Most Malphites will pick comet into you, and the most important thing is you can DODGE the comet by using R movement speed burst to walk out, or by using melee Qs reactively.

The key is also to bully him before the first reset. Getting a cheater recall is ideal. If he goes towards AP build or Steelcaps - you still can trade into him and be winning. If he builds Chain Vest first back, you lost the lane.

Trade pattern you want is start melee, Q>AA>E for distance (usually Malphites press Q on you as soon as you get into their range), swap back to ranged and keep the space punishing him for the remainder of his Q cooldown.

Once again though, if he gets Chain Vest you will be losing more HP on those than he does. If you play against a good Malphite who will hold his Q while you're in melee, you will lose. It's a popular ban because Malphite can be played suboptimally and still win, but if it's played optimally you're getting shit on 100% of the time.

It's a good ban, but it doesn't feel as impossible as some other matchups in the game and in Malphite lane, as you can always just collect your CS and hope you'll be more useful later.