The biggest probelm with Malphite is defining his role. He has such a versitile skill set, with lots of skills and items that can work on him, but it almost feels like the old adage "Jack of all tades, master of none".
One of the biggest pitfalls with him in my opinion is trying to play him as a full on honest to goodness tank. The reason being is that he lacks the one thing that every tank must provide his team, a reliable way to mitigate incoming damage to himself AND his teammates. Sure he can make himself very hard to kill, but his lack of ability to keep his team alive in most real game situations just makes him less than ideal in teh role of a true tank as i have defined it.
There are exceptions to this, like when the other team stacks lots of rmelee rightclickers, but those games are rare...though no one can likely shut down a team of that nature better than Malph. The AS redux from his GS, Frozen Heart, and Omen are epic.
One thing he does excell at however is being a very strong Anti-DPS champion. When used in this fasion he acts like a battering ram more than a shield for his team, and smashes his way right on top of the enemies biggest DPS threat. His ult is perticularly well suited for this, and he has an unparrelled ability to disrupt channeld DPS ults from the likes of Kat, MF, Twitch, and others. If built properly he can decimate the other teams strongest DPS threat, either killing it outright, or forcing them into a position to flee and die on the run. He can usualy survive long enough to wreck enough havok that his team wins, and he may even be able to escape alive if they join quickly enough. This only works with a team that is johnny on the spot once you dive in...otherwise its feed city, and thus makes him an unrelaible choice in normal solo que as good teamwork isnt always easy to find.
If you focus on this as your ultimate late game roll in team fights, you will defintly make a positive impact. To this end it pays to build a little tanky so you can survive the intital retaliation of the enemy, but it also pays to build items that give you the most burst damage possible. Here are some items that i have been using to great effect when playing this style of Malphite.
-Haugnting Guise + Sorc Boots: Early game i like to pick up one or both of these items depending on my prey. The magic pen along with maxing shard as early as possible makes for a very powerful nuke that you can deliver at will to the squishiest enemy in your lane. You will need something to mitigate your mana, and i use runes and masteries to for mp5 myself.
-Glacial Shroud-->Frozen Heart: This item is key because it gives you mana, CDR, and AR for a decent price and makes your GS a very disrupting skill when paired with the aura of FH.
-Abysal Sceptar: This along with my passive scaling and MR/lvl glpyhs is usually all the MR i need, with Mercs being thrown in if i need them. The AP and MR redux is gravy for adding DPS to your burst.
Hourglass: This item provides a double dip in DPS, giving you both AR for your GS, and AP for your ult and Shard. The active can also keep you alive long enough after engaging for your team to save your rocky butt.
Sheen-->Lichbane/Trinity Force: Either of these items provides a solid chunk of burst damage to Malphite as well as several other defenisve stats, and sheen is great if you can afford to grab it earlier for the burst it provides you in your opening combo. I belive lichbane with Hourglass and Sceptar actually give you more damage in my final build, but its pretty close until you get my last item.
Void Staff: In order to pick this up you need to sell HG. It will easily replace the 20 Mpen you lose by upgrading you to a wooping 49% MR redux if you paired it with the 15% mastery. This is only to be had in extream late game where most champions will have some investment in MR. It also makes your Lichbane hit like a truck giving you somehting like +350 AD a pop.
Malphite excels in dealing magic damage, so it makes sense to grab items that enhance this. A few items i would consider for thier synegry with this playstyle include...
Thornmail: Deals magic damage back to enemies and give you cheap AR.
Sunfire Cape: Deals magic damage aura and pairs well with Abysal sceptar, while giving you more durability.
Wits End: Deals a solid magic damage proc and grants MR, a stat thats often in need. It also grants a very nice chunk of AS, and a little known fact is that Malph is one of the best champs in the game when it comes to how well he scales per point of increased AS. This makes dropping towers much easier and makes your Burtal Strike active bonus damage more effective since you get more swings in during the duration.
This build has been working wonders for me if i can manage to get to the mid/late game. Laning can be rough against the wrong enemies, and getting zoned has been an issue when i get stuck in a double melee lane against too strong ranged champs. However, late game i really shine if all hasnt been lost and my team is with it. Good luck on finding a playstyle for malph that suits you and works well.
Stay Rock Solid,