Skarner is a very strong AP champ that can jungle pretty nicely thanks to his e spell. I've had quite a lot of fun with him and found that with a good amount of AP, AS, and HP he can be a really strong champ. His spells don't really help him as an AD champ as much as they do as an AP, especially his shield so I decided to practice him as only and AP champ with items like Rageblade and Nashor's tooth to take full use of his passive and also help his damage output quite a bit.

My runes fit the champion well imo, cd reduction per level is really good for late game because your damage from your q doesn't scale well with AP so a good way to increase your damage output with spells like that are by increasing the amount of times u cast it. So technically without blue buff i will have 8.1 percent cd reduction plus the 3 percent from the masteries and then the cd reduction from nashor's tooth. increasing ur damage output over time by quite alot. It's great for ultimates like skarner's that have a large cooldown because it will save you in clutch moments in the game. I will talk more about that in the skills chapter.

I chose these items because the synergy with the idea of him being a hardcore ap champ with extremely low cooldowns due to his attack speed. if you can halve his e's cooldown with the rageblade boost and the nashors tooth even without the boost from your shield spell/ attack speed/ movement speed boost.

The starting item should allow you to jungle if you max your e and use the correct runes and masteries, but you will have to get a leash on blue buff. His jungle is a bit slow but I find it to be worth it in the long run. u just have to make sure to get red within the blue buff or you wont get it without running totally out of mana. late game you can sell the philo stone happily, it will be worth having the philo stone until you have enough for an item that you really feel you need and dont have the space for. I recommend always getting rylais and nashor's tooth for they help you the most. A rabadon's is also a must have item just because you want to have a huge amount of ap, the gunblade isn't necessary but I find the slow on it is very useful after you ultimate someone and will nab you kills early on better than a rylais would so getting it before a rylais is a good choice if you plan on getting it at all. The rage blade synergizes well with his passive and the idea of him being an ap champion but its not necessary as nashor's tooth does the job better in my opinion. The boots help you more than a set of sorc boots would mostly because it helps you recooperate your ultimate and your e and w spells so well. Lich bane is a highly recommended item as well just for the proc and the ap. the movement speed boost is great too for chases.
So if this is TL:DR here's the short side of it:
Early game: regrowth pendant into philostone--boots of speed--sheen--bilgewater cutlass/(depending on time)berserk boots-- hextech revolver-- gunblade or large rod if you want a quick 80 plus ap :P-- finish rabadons--finish lich bane/or finish rylai's depending on how you feel you are managing the damage of the enemy team/ build nashor's tooth. if you feel you need some survivability rather than damage take off the gunblade or the lich bane.

Your passive: Excellent spell, can drop your spell's cds, especially your ulti's cd a large amount if you farm up a bit and build attack speed.

Your Q: A weak damage out put early on but scales well enough to help you farm and kill enemy champion. My main use for this spell is to proc my rylai's slow constantly and my lich bane proc everytime it comes off its cooldown. This gives you an enormous amount of burst put together with your e and your ultimate. Since the activation of this q doesnt make you stop to do an action you will always be able to keep up with an opponent that has been slowed by your rylai's.

Your W: It's a very good survivability spell playing ap, this spell with a full build will save your life... no doubt about it... you are boosted in speed, you attack faster, meaning you proc your passive alot faster, meaning your cooldowns for your shield are dropped an enormous amount to the point where you can constantly have it up if you don't get damaged. overall I really, really like this spell. I rank it up as a second priority behind my e, i also get one rank in my q before i get my shield and then another rank up in e before I get my w rank.

Your e: This spell is your main source of damage and you HAVE to hit it to keep up with enemies you want to kill unless you want to waste flash to get to them. your w spell will get you to catch up to them enough to get off your e so you can hit them more and get up the e and w up a second time. your E is also the first thing you rank up and allows you to jungle pretty well. dont run out of mana and use this spell every time you get the chance to while jungling and you will do well. its not a complicated spell but it helps you out quite a bit, early game especially.

your R: OMFG this spell is useful! I'm sad I cant flash with it on but oh well I will sometimes need to flash to get to the enemy to use it anyways. I used the flash/ult combo a few times to kill an enemy veigar before he could cc my whole team. soooo funny. It also goes extremely well with your passive and your cooldown reduction and attack speed. just ult a carry or someone that is causing you and your team trouble and there you go, you have killed the enemy team's carry. use your W right before you ultimate to pull the enemy into your team quickly and shield urself from oncoming damage. after you use the ult, use gunblade proc if you think he has a chance of running away and e him and q him.

Skarner is a useful champion, some would say underpowered now but from my experience I find him to be quite useful and powerful on a team. You can play him with alot of resists too and reduce the damage you do but increase your lifespan to get and keep your ultimate up during a teamfight if that is your trouble. once again, the lich bane or the hextech are good things to take off if you cannot survive a teamfight. Have fun with him and UPVOTE MEH.