As the following looks is just what it suggests malphite as major caster and a has that sustainability to boot. Not only can you easily support your team into winning a team fight but typically become the major roll in anything when in game play.

I will briefly cover runes, skill work, farming, and hope that you will just take my word for it seeing as im sure you havent seen a caster malphite that often anyways...

The runes are set into place for reason that his ground slam is worked off 1/2 of your armor leaving out any ap that you obtain in the possess. As for the rest of the runes; well your a caster arent you and your ulti does give you 100% of your ap. nuff said

As for the items and the game play. Way early on your gonna notice your pretty squishy and your relying on your shield to take up a good ammount of dammage and to top it off you have no mana! HAAA, but dont worry soon enough youll get the ball rolling as you make the first part of Frozen Heart. Following up with shoes. Now this is where youll have to make the hard choices.

My items are as follows and i try to always keep it in that order, but not every game allows me to do so. The major items would be both scepters and allowing yourself to get what you need in what time you have to get it allows you to be very versatile.

As moves are concerned always try to make your ulti hit at least 3+ people and combo up with the rest of your moves as follows: R, W, E, Q. Afterwards if most of the group you just attacked hasnt either died off or trying to run already have your E ready as your W is about to wear off and become on cool down, following up with your Q most people cant even stand how BS your slowing ability will be.

Creeps Monsters:
During mid game play abuse your W combo E boosting your armor for a short bit and allowing most of the creeps to die off with E if they havent already died off from auto attacks.
Dragon:
Having early game dragon is sooo nice and with a malphite who has his first part of Frozen heart really helps when your wanting to really help the team out. Not saying you can solo Dragon cause you will need on of your real ap or ad chars for the dammage, but they always come if you gladly give a ping and say FREE DRAGON! lol
Barron:
Once being 15 mins into the game you should have your Frozen completed and usually around the 22-25 min mark i can ask for 3 people and me to take a barron, MASSIVELY abusing the slowing effects for his attack speed making sure that your W combo E is your only moves your gonna be doing here and really hoping that the other team hasnt caught on.