Congratulations on joining the ranks of those most rock solid of LoL players, the Malphinators. I've been playing the big guy since the game released and here are just a few tips for his current iteration in S5.
Know your role...Malphite will always be a semi ok pick due to the potential of his ultimate in team fights. That doesn't mean he is always the best pick. Learn to optimize your in game potential by taking a role that works for the match at hand.
As one of the most verstile champs in the game, Malphite is a bit of a jack of all trades but master of only one. Here are a few options that work well...
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Anti-Melee DPS: No one shuts down an enemy team that over commits to melee AD DPS like Malph. NO ONE! if you get lucky enough to pick Malphite into an enemy team that has to much AD, take Frozen Heart, Randuins Omen, Thronmail, Sunfire Cape, Ninja Tabi, and a luxury item of your choice. The sheer amount of AS debuff you can drop will ruin any plans they have of face pounding your team.
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AP Assassin: If the enemy team has a dangerous AoE channel Ultimate like MF, Kat, Twitch, or someone similar...be the rock that ruins their day. Build as a bit of an AP assassin off tank with items like Hourglass, Liandries, Abysmal Sceptar, and Iceborn Guantlet. Anything with AP and defensive stats. Then stay close to your team and be Johnny on the spot the moment you see the enemy channeler hit their wind up. Your ultimate will cancel 90% of their damage and your bust will make short work of them, allowing your team to clean up.
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AD Bruiser: if your team is lacking in AD, or you notice the enemy ovwrcommit to MR, you can easily become an effective AD bruiser with a small investment In one of a few items. Malphite was a bit of an AD beast back in the days when his steroid was a 70% boost to his AD...but even at 40% it's nothing to sneeze at. Taking an item like Hydra, Trinity Force, or the new Brutalizer are all great ways to give yourself a nice spike of AD to your output. Balance one of those with your normal tanky staples and you will be able to wave clear, jungle clear, and take towers down like a wrecking ball much better than a traditional full tank Malphite, while still taking plenty of punishment to the chin.
Final tips...
Be aware of your shield. It is your stustain both in lane and the jungle. Always let it recharge before returning to battle.
Try to engage with your Q during skirmishes and teamfights as opposed to your ultimate. Use the speed to close into groundslam range and your tankiness to soak poke and CC...once the enemy has engaged or retreated, then land your ultimate, catching most of them while your team mows them down, or snagging the slowest enemy who is out of position while his mate flee in terror of your might. Also allows you to chase flashers like a heat seeking missle if you save it instead of open with it when catching players out in jungle or roaming alone in lane.