Given the opportunity, the Gunner has the potential to be one of the single most devastating units in the game, able to take at least a Medium Titan from full health to death nearly by himself, but he has certain quirks that must be understood about him. His opening damage when he starts firing at his single target is weak, but given a couple seconds, he will do more and more damage until his RPM goes through the roof.
There are two instances where the Gunner's RPM will reset and start again with the weak damage. The first is when he changes targets, which means the Gunner falls short against massed infantry since he can't bring his full damage to bear against each target. The second is when the Gunner moves, for any reason. If he has to shift position to hit a moving target, the damage resets; if he gets shoved by an enemy drop pod or Titanfall, the damage resets. To beat the Gunner, you have to disrupt him somehow.