this is a guide for an offtank, meaning that its not for doing insane damage, but to do some sustained nice amount of damage, from a very tough source.
malphite is a great initiator, and hes skills syncs with ap, but he can also be used as an ad, all the bonuses from hes abilities, like stealing movement, increasing ad, and stun from ult is very effective on an ad, so malphite got it all, cc, survivability, hart hitting, and a nice attack animation/sound

some people may say that im ******ed to create an ad build for malphite, but it actually works quite well, here ill give the reason why.

why not AP malph if you are going for more damage than normal tank build?
An ap build for malphite does more burst damage than AD, but you got mages like anivia and annie to do that anyway, in the long term, ad will do more damage than ap, and since you are a offtank, you should proberly go to long term.
and the damage from your abilities does some nice damage anyway, no need for ap to improve that damage, rather just get damage from ability lvl, and then use their special effect.
and your BRUTAL STRIKES gives u a nice amount of increased damage, as well as some aoe, that combined with an TIAMAT will give nice amount of AOE

Why not get an another ad offtank, like irelia?
Irelia is a very nice offtank aswell and in some situations i would prefer using her, but she dont have a 2 sec AOE stun innitiate ultimate, and vs other AD malphite is way better. and due to your BRUTAL STRIKES, you do more AOE if you focus right.

if you need the offtank to be a jungler, ive added a section for that later on.

Pros:
very tough
good early game do to passive
nice sustained AOE damage
heavy cc ultimate
good chaser
can jungle

Cons:
not as high burst
not good 1v1
slow jungle
mana hungry

Armor pen red = nice damage
Mana pr5 yellow, you are very mana hungry
MR blue, u are a offtank, u need survivability
Flat HP quints = syncronises with your passive, and nice early game

i go 23/6/1, with improved ghost, because you get survivability from your items, and u get some nice damage this way, and still increases both armor and MR

i go for an early sunfire, to increase farming with GROUND SLAM, and it gives both HP and armor, that greatly increases your sustainability, and a nice damage from its passive. then i get a Frozen mallet to inshure more kills for my team, as well as more HP. now its time for some damage, and you need that quick, so i get a Atmas impaler, not only its cheap, but gives both Armor, crit and nice damage for offtanks/tanks with all that hp you get.
now i pick up my Tiamat, this is often seen as a bad item, and in most cases it is, but for an ad malphite, its awesome, increasing damage by a fair amount, and giving you more aoe on attacks, ending up in your aotu attacks actually does more damage to enemies around your target, than your actual target, and if you then focus on the target with less armor, the enemies standing aside your target takes a VERY high amount of damage, and you deal lots of aoe, so go for the right focus, and if enemies are standing close, they'll go down.
the last item is very situational, if my enemies got lots of magic damage, i go for a Banshees veil, if they got some heavy melee damage, ill get either Thornmail or Randuins omen
but if you just need to do more damage, grab a Phantom dancer or Infinity edge, what suits your style the most, id prefer Phantom dancer, as i like the sound of malphites auto attacks, and you might just not have enough crit chance for Infinity edge

in a teamfight, you have to focus the enemy carrys, you got the survivability and abilities/damage to do so. your strengh is then you focus on closely grouped enemies, if they group up on a squishy with low armor, as your auto attacks does more damage to targets around the target than the actual target. and then you got something that no other oftank has, a ultimate, that not only a great initiator, but does heavy AOE damage, even without AP, and a very usefull stun, making shure your enemies cant flee from your AOE strikes. then you stand close all your enemies AD carrys, use your GROUND SLAM, and disable them, for an easy massacre. use your SHARDS on enemies that tries to flee, and chase them down.

Im not a good jungler, so ill make this one short.

start at wolves, only get the big 1, then go for blue, and get a pull by your teammates blue, normal blue jungle route thereafter.

SKILL SEQUENCE:
E,W,E,Q,E,R and so on

grab a fast lantern for items, if you are jungle
and for summoners, ghost and smite

Malph is very intresting, he does nice melee damage, and hes abilities does nice base damage, so he is a very effective offtank, and with a ultimate, you can turn the fight, but only use your ultimate on squishies, and only then u can hit more than 1, in early game, u can go and use your ultimate on that low hp tower hugger.

i havent made any play style throughout the game, there are other guides for how you farm, jungle or gank, so its a no need.