Malphite is weak in the early game. Try and trade and play aggressively to delay his power spike and his ability to scale. Once he has an item, you have to be more cautious when trading, but up until then, he can be heavily abused.
The shield from Malphites Passive offers more security in lane. Try to poke and harass him as often as possible so he is unable to get the shield and protect himself.
Once level 6, Malphite may start to receive ganks as his Ultimate R is a great engage tool. Avoid playing recklessly and always keep an eye on the minimap especially if you’re pushed up in lane.
Avoid grouping tightly together as it will give Malphite a great opportunity to land a 5 person knock up with his Ultimate R.
Do not look for fights in the jungle or around the Dragon or Baron. This is because teams will generally be grouped closely together- which allows him to get a good Ultimate R off. Instead, try to fight in the open in a lane.
Poke and disengage are Malphites biggest threats. If you can, try to poke him down as much as possible before a team fight occurs to make it near impossible for him to engage without dying. Keeping him low will force him to recall.
Malphite is at his weakest in the early game. Try to set him behind to delay his next power spike.
Once Malphite is level 6 and has his Ultimate R, his kill threat in lane increases and he can start being more aggressive.
Team fights are where Malphite really shines as his Ultimate R is an AOE ability and can knock up multiple people.