TLDR: Build tanky, survive laning phase, trade mid game, look for team fights late game and go for objectives.

Malphite is a tank. He is meant to be built as a tank. Hybrid AP/Tank builds for Malphite aren't good because he doesn't scale well with AP as much as he does with armor. But, whenever Malphite is fed and builds his core items, he is free to branch into the wonderful fine arts of AP.

During laning phase, you want to take minimal poke and sustain in lane for as long as possible until you're able to reach your first purchase (ideally Glacial Shroud + boots). Your potions should keep you alive until you back. When you come back to lane, you should be able to poke out the enemy laner so that you're then able to auto-attack minions without wasting abilities on them, and you're also able to take short trades with the enemy as well.

After laning phase, look to support your team around objectives. It's important to lead your team into winning team fights by utilizing your ultimate on carries such as fed ADC players or mid laners.