Okay, how I play Malz. First thing you need to understand, Malz has 3 things he can do:
1. Hold a lane against the entire other team for about an hour
2. AOE Damage during team fights, brutal
3. Zap 1 enemy and absolutely stop him and kill him during team fights
Important note: Don't feed, by that I mean you should not die more than a time or two.
IMHO these are his best uses, get some kills, lots of assists and don't give up the hoochie.

Pros:
-Ult is insane
-Farm like a beast
-Hold a lane forever
-Gain Mana while farming

Cons:
-Squishy
-Requires you play smart
-Ult can be interrupted, never let this happen!

Runes are so badly misunderstood it drives me insane. You need to use the most effective runes for the slot. Simple math renders these concepts, don't just stack AP, it is a waste:
*Please note, math is based on what you can buy
**Movement speed is basically the most powerful stat in the game, sorry but it is, only Quints are helpful here and they are super powerful, 1.5% is amazing. Boots are not good math after +2
***Also note, you need to think levels 1-5 vs. Late game 18
****I use a simple ratio, compare the affect of the Quint to that of the M/S/G and you get a ratio. The higher the ratio the better the rune. Example is AP(Force is better at 18th) Mark&Glyph gives 1.8, Seal gives 3.06 and a Quint 7.74, so the Glyph is a ratio of .39 and the others suck. The best ratio is about .5
*****Now we factor in what is easy to item and what is hard, example is HP5 vs. MP5, in the beginning you can get 15 HP5 and like 7 MP5, so MP5 is worth double HP5 in the beginning. So 9 runes of Replenish(.41 MP5, .33 ratio) would give you 5 MP5 right at the start, then you could buy some HP5 and lane soo much easier

Marks: MP(Mag Pen) is the only possible mark(.50 ratio)(AP would be like .2)
Seal: You could do MP5 or HP, yes HP, how many times I need to say this, if you die you feed
Glyph: AP seems obvious(.39 ratio) but cooldown is also .39 and AP is much easier to item than CD.
Quint: I think 4.5% move(which takes you from boots of speed 2 to boots of speed 3 effectively) is so dang powerful. I think I live 5 times every game where I would have died and I get maybe 3 more kills during that game just on these Quints!

I think this is for the most part a no brainer, Damage and Mana to do the damage with.

Now the way I vary from most people is I think Flash and Ghost are very helpful. So I buff these. And I flash to kill and run away. I often use ghost to get back form B.

So, 9 in Offence, pure magic Damage and Pen
Utility: 1. Ghost and Regen, 2. Get the mana, you level way faster than anyone because you kill minions, and I care about late game 3. Regen and Neutral Buff (OMG Blue is sick) 4. Move and Flash 5. CD and 6. CD

If you get time dead please don't read this guide, you will not like it.

Items are clearly the deal. Now we get to playing style. Do you spam your abilities? Do you let yourself take harassment damage? Are you crazy? I am careful for the first 3 levels, focus on last hit. I never let my mana stay full, but I never go low in mana, you must have a silence to stop a gank. So...
1. I get boots and Health pots first, if not these, the Ring is so amazing, regen, 100 hp and 15 AP, holy cow that is awesome
2. Tears are amazing for you, if you are being fed, get your Mejai's and reverse it
3. Get your boots, at least level 1, speed kills
4. Unless you hate stacking, get the Mejai's
5. Now we are ready to lane forever, get your Hextech and you can feed hp and mana all day long
6. If you are doing well, you get your Archangel's and Will one after the other. Always go back when you can get one of these
7. Now you are just getting brutal, Deathcap and it is GG, another Archangel's and you are a beast
****Special note, until level 10 ALWAYS have a health pot with you, always. Buy your share of wards and when you are full buy the elixirs just to get you over 1000AP

So I build to be able to do massive damage and lane forever. You can hold the lane, alone, while your team either back-doors them or they take 4 guys and rip down a turret. Think of it this way, you can hold mid against 3 peeps at level 10-15 for a long time if you are careful leaving your team to pound on them 4v2 somewhere else! Do your job!

If they are heavy tanks, change your order to Null Zone over CotV, because it rips tanks down.

Your passive is amazing, 10 Mana back on a kill, time your visions right and you can suck up 30 mana so it costs half as much.

So, your main weapon is your silly Visions, it is sick. Early game you must weaken the creeps first, I actually attack all 3 of them and soften them up then feed off them for mana with my Visions. Time it right and you can drop it on a champ, WINNING.

Now, CotV is a silence and crazy damage. I use it to silence then pound them to death with my other abilities and suppress them with your Ult. Think about it, silence is golden in most cases.

Your Ult, one of the most powerful weapons in the universe. Created by Einstein to obliterate the Nazi's, this thing does it all, IFFFFFFFF you time it right. Don't let your Ult get wasted! Don't get stunned, don't use it on a Ring, don't use it on stinking Trynd. There are 3 things you can do with it and damage is NOT the most important one:
1. Grab and hold someone so they can't hurt anyone. Do this in a turret range and ouch. Do it on your CotV and Null and brutal.
2. Waste someone elses Ult, man this is cool
3. Save yourself or one of your mates, when someone is running after a fight, drop a CotV to silence and then use your Ult to crush them.
4. Damage. Yes it does damage but if you ever use it to just damage and not either Kill one of them, or Save one of you, please give me back my guide and play a different toon.

I use Flash and Ghost, almost excursively. I do NOT like to die, I think giving up 400 gold is stupid. I think in a team fight an ACE is a stupid goal, a good goal is to kill 1 of them, get 4 assists and then leave. This is worth about 1200 to your team! Another good fight is kill 3 of them and lose 1 of yours, which your team should also get 8 assists if you can manage it. This is an excellent team fight. It is all about the money guys. Get more items and kill them.

Okay, this is how you play Malz late game:

*You use your CotV to weaken their harrasing pushers.
*You wipe out thier creeps in 3 seconds from a safe distance
*You ALWAYS put Null under your tanks when he engages! Always!
*You Ult their Melee pusher, don't try to get a mage in the back, that is death. Drop Visions on a melee when he comes into the fight OVER your Null then ULT him. He dies from the Visions and the Null, not the Ult. But it is too late, he just stood there for 2.5 seconds doing NO damage. No damage is the important part, he is dead, and he did nothing! Never try to get to the back squishies, kill Jax or Xin or Kat.

If you ever attack a tank, except Null zone which is really there for the enemy melee, you need to go out side and Null zone yourself.

Now I want you to repeat after me, RUN and use your health pot, I told you to always have at least one. Then COME BACK!!!! Don't just run, you are basically stepping back when your abilities are on CD. You do about no damage without abilities. Never die OOM or on CD. Just run. If you can come back to drop a CotV to silence them that is awesome! And get a Zone down might scare them away. Now a visions on the creeps will hold them off too.

I have some things I want to say, you will hate to hear them but tough cookies:

*****Unless the game goes soo long that everyone is 100% full, in which case Xin, Kat and Jax win, it is NOT turrets that win the game, it is good team fights, $$$$ and creep pushes. If your team does good team fights, you win 95% of the time. If your team has more money, you win 75%, if your creeps are pounding on their turrets, not the other way around, you win 50%. People spew that turret **** all day, but the facts are if you have more turrets, you normally have more money too and your creeps are pushing them. Forget the silly phrase and kill their champs, get the money, then bring down a turret. Then go back and shop, rinse and repeat. 1 kill and no deaths is worth a ton!

****In the beginning of the game, you want to last hit minions, haras the enemy AND do it near your turret. This is a concept I preach to new players. The trick is not to auto attack for the heck of it, only do it to KILL. in the beginning game there are 3 melee and 3 range creeps. If he pounds on his 6 and you only last hit your 6 you will be closer to your turret(meaning safer), have more money than him and the same experience. This does NOT mean let your turret take damage. But just think about it. Why does Ashe waste a volley to weaken creeps? You care only about 3 things:
1. Last hit creeps
2. Harass
3. Don't get freaking ganked

*Tanks initiate, not squishy mages. OMFG what is wrong with some people? Why are you so greedy or impatient that you can't wait for the turtle or the ninja to go first? Is it you are so impressed with your burst you think you can kill all 5 of them? Well you can't and you will die and you suck. This is a good team fight:
*t-.5 sec: Before the fight, you have visions on a creep!
*t 0 sec: Your tank goes in, their tanks jump in
*t+.5 sec: You drop Null on your tank, starts it's CD, you need to get it down ASAP!
*t+.75 sec: One of their melee comes in, they always do, so impressed with their burst
*t+1 sec: You CotV, silencing their Melee and tanks who all of the sudden can't do any super abilities, dang that sucks for them. (BTW, you now have done damage to like 3 of their toons, assists rule!)
*t+1.5 sec: You cast Visions on their silly melee, NOT a mage in the back.
*t+1.5 sec: Start your Health Pot going, even if you are NOT damaged!
***Note a voidling is out there now, or maybe 2 of them
*t+2 sec: You ult that silly melee who has visions on him and is in Null. He is screwed, he is taking about 600 DPS and the voidlings turn on him and will follow him even if he lives.
*t+3 sec: You are weak now, taking AOE and some mage fire, flash out of there or run
*t+4 sec: Turn around unless they are chasing you. If they are chasing you this is the BEST possible situation. They are wasting TIME while they run at you. No damage being done. Your CD's are spinning.
*t+6 sec: Your friends are running too now, but you have CD runes and your CotV will be back up soon, Drop it ON your running buddies, it will go off on top of the mage chasing them and silence them and do nice damage
*t+8 sec: you drop visions on one of those mages who is running from you now, silent and QQ'ing to his friends on vent who are already dead, about how tough our tanks are, but our tanks are NOT tough, their melee and tanks were silence and standing in Null. Now he feels visions, 2 voidlings and an auto attack, he lives about 2 seconds longer.
*Your health pot has healed you for 75 HP, which can save you from an ignite.
*Your surviving tank is still there, shocked he is still alive, bragging about how good his build is when in fact he is alive because you silenced, suppressed and killed the melee who jumped him making it a 5v4 fight. That tank uses a taunt and saves your butt from a mage who is now off CD.
*Run again, hide, call your mom, live. Your wad is blown.

****Pay attention: There are 2 things that matter in this game: Burst or Tank and getting to level 18 first
-Burst or Tank: means if your burst is better than theirs, and you have mana when the fight starts, YOU WIN. Or, if their tanks can live through your burst, and you aren't sneak enough to focus their melee or mages, you LOSE!
-Level 18 is achieved either by kills or creeps. If you can stay in lane and pit stop less you will be higher level. I never have the XP mastery and I am always the highest level in the game because I only pit stop when it is the best time:
Think of this: You are against a Cait, sure she is hot, long legs, tall, etc, but she also hurts. So you stick and move, harass with your CotV and last hit creeps. She has to go back so you should too or you should Gank. When do you go back? I will tell you, kill all of her creeps, safely, then run back 30 feet and B. Now this is what happens, she gets back before you, and goes to town on your creeps, killing them like butter. But her creeps are coming in a snowball. So you hustle back, get there JUST before your turret. You blow right in, but you attack the creeps, not her. You throw it all down and you kill many creeps and get all the XP back she got while you were back and you get big money too. Also you are near your turret so she can't hurt you or gank you. Then you push back because your visions farmed like 100 mana for you and soon you are right back on top of her smacking that booty!

More to come later, I am tired. Please use your head, not your ego and stop being so rude and cocky. Try saying something nice out there and you will have people to run with instead of ELO hell *****ing at other peeps while your raw dog some randoms. Give me 4 fun people, even if we lose, and I will have fun. Running randoms with 4 other people who all want to be carry and that sucks, and you lose, and you deserve it.