This is a quick humorous guide to a farming master. I played around with it on bots it was pretty fun and funny. I'm not sure how this would go in a normal game, it MAY work I'm not sure. All I know for sure is people will call you out for trolling if you want to use this haha.

Pros:

AoE is like 168% from Tiamats and your W
Not bad for team fights.
Awesome farm, wipes large minion waves in one or two hits.
Decent health and mana regen from Tiamats.
It's just fun seeing minions being melted.

Cons:

This won't help you dual.
Lacks tanking qualities that people look for in Malphite.

Runes....

I don't really know go Armor pen. Marks, Flat Armor Seals and Flat Cooldown Glyphs

Armor Pen for your autoattacks,
Armor for your Ground Slam.
and Cooldown for both your Ground Slam and Brutal Strikes.

Full Attack Runes could work also.

Go with 21 - 3 - 6

You get full attack from Offense Masteries, +6 armor from Defense, and added Ghost and Regen + a lil' exp from Utility.

The Key item here is:
icon=Tiamat size=64
Tiamat

Its passive is not unique so it can stack to give you 100% splash on your melee attacks.
Which also stacks with Brutal Strikes which gives another 68% at max lvl.

I only got two when I tried this, I'm sure you could get more. As far as I am concerned the items add 50% of your damage separately so it would be like attacking the surrounding enemies 2 for half damage, or just once for 100% damage, but you get the point.

I would fill the rest of your inventory with:
-> -> ->

That should give you enough damage, crits and speed for a devastating rock-man.

I don't know when exactly you are allowed to buy what, so I just put Brutal Strikes for the entire sequence.

That basically sums up the skill sequence though, get Brutal Strikes rank 5 first then Ground Slam and Seismic Shard last, obviously leveling your Unstoppable Force whenever possible.