Disclaimer:
This is a troll-ish build for Malphite that is fun in lower ELO. It can be good in a mid-ranged ELO, but that's 100% team based and you would have almost always ended up better off using another build/champion.

Note:
This is not a full-on jungle build. I start in a 2v2 lane, preferably with a champion that has a decent amount of self-sustain. I leave lane to jungle after first buy (hopefully around level 5-7), returning frequently to GANK after level 6.

In short:
DO: Rate this build on fun-factor, ganky AP burst ownage, and the fact that you're doing something a little different on LoL for a change.

DON'T: Use this for ranked play, expect to be consistently good with this build, expect to be good at all with this build, expect anything from this build PERIOD (except a little fun).

Pros

• Unstoppable Force is beastly. 400 damage + 100% AP at max level!
• Seismic Shard + Blue Buff + Lich Bane = Profit!
• Decent jungler with Ground Slam, Brutal Strikes, and your AP-enhanced Seismic Shard

Cons

• Not as viable as a tank since we're not using your typical tanky Malphite build.
• Mana/CD dependent (and we didn't build CDR or Mana Regen!) -- needs to keep Blue Buff as much as possible.
• Not a viable build... and you might get yelled at and/or cause your team to QQ profusely...

Take Seismic Shard level 1 to try to secure a kill. This skill will later be used as a source of decent damage, as a chase mechanism, and to trigger your incredible Seismic Shard + Lich Bane combo!

Take Ground Slam for the next two levels to start dealing more damage. With armor masteries, runes, items, etc. -- this will be your most damaging early game spell. Later, this will be used to trigger Lich Bane/ Sheen procs, slow attack speed, and deal somewhat decent damage.

Brutal Strikes is leveled last, but we take a level around 7 to jungle a bit faster.

Unstoppable Force is leveled whenever it's available. At 200/300/400 damage (+100% of AP), this will be very strong at mid-game.

Skill sequence is flexible, I've posted roughly what I recommend for this.

Smite will be used early on for farming creeps and getting the +5 gold boosts until you start jungling. Since you have Smite, don't be afraid to go bottom lane and start at the golems with your partner to get a head start. You should be able to take minimal damage with your Granite Shield.

Flash is the most versatile spell in the game (the end).

If you find that you have trouble with mana throughout the game, you may consider taking Clarity.

Don't do it!

The quick explanation is that 9-21-0 gives you magic penetration, enhances Smite, and gives you some additional defensive stats since you're AP/offtank-ish.

I find that starting with a Cloth Armor and 5x Health Potion is the way to go for more lane sustainability early game which you're relying on to get to the jungle faster!

On my first trip back to base, I like to try to buy Boots and a Chain Vest.
The Chain Vest is awesome for Malphite early on. I'll explain:

• 13 armor from Base Stats
• 18 armor from Cloth Armor
• 45 armor from Chain Vest
• 6 armor from Hardiness
• 13 armor from Greater Seal of Armor

Total Armor: 95

That's +45 damage to your AOE Ground Slam and enough armor to hold you over until mid-late game! Ok, even if it's not that much, I like it. You can turn that Chain Vest into your Zhonya's Hourglass later on.

Where were we???
Cloth Armor + 5x Health Potion for lane sustainability
Boots to run faster!
Chain Vest see above
Boots for the situation you're in ( Ninja Tabi is nice; Mercury's Treads for enemy CC)
Sheen will turn into a Lich Bane later and will proc extra damage now for jungling/ganks.
Abyssal Mask if you need magic resistance; Needlessly Large Rod if you need damage!
(The other one)
Rabadon's Deathcap for lots of AP!!!11
Lich Bane at this point, you're dealing tons of damage with your Q and R -- then you add 400-500 damage per proc.
Zhonya's Hourglass
Elixir of Brilliance - when you have spare cash, the AP is useful!
Oracle's Elixir - if you're roaming/jungling a lot and surviving most battles... grab one of these for map control. (READ: don't buy if you're dying frequently)

After Chain Vest and (optional) Ninja Tabi, you'll jungle with ease! I like to buy a Health Potion or two when I head back to base to keep my HP up, but you'll end up absorbing a lot of damage with your 90+ armor and Granite Shield.

After you have Sheen/ Lich Bane, make sure you stagger use of your spells on larger creeps to proc more.

Save Smite for Dragon/Baron or to secure Red/Blue buffs when enemies come near. Especially after you get Lich Bane.

Your gank sequence is:
Unstoppable Force -> Brutal Strikes -> Ground Slam -> Sheen/ Lich Bane -> Seismic Shard -> Sheen/ Lich Bane

Brutal Strikes early on will increase your armor when you first dive in to absorb more damage and deal more with Ground Slam. Seismic Shard as needed to slow, catch, and secure kills. You're tanky enough to dive as well!

Seismic Shard has a duration of 4 seconds and, with 40% CDR, is available every 4.8 seconds. You can initiate ganks with this, if in range, to ensure that Unstoppable Force hits. It'll be off CD by the time you need it for securing!

At level 10, you'll have level 5 Seismic Shard, level 3 Ground Slam, and level 1 Unstoppable Force. Assuming ~90 armor and ~80 AP, your combo (not counting procs) will deal approximately 783 magic damage in ~1 second!

This is a fun build for having fun! Early to mid-game ganks with this Malphite can be completely deadly and will help you get early towers (which means more gold & a wider level disparity vs. the enemy team!)