So .. after hundreds of WZs on my Merc, Assassin and Marauder I have a simple idea for giving Arsenal Mercs more of a chance in PvP without bring overpowered and (equally importantly) not requiring any new abilities or animations.

Power Barrier is replaced by Targeting Countermeasures

Power Shot and Tracer Missile have a [50/100]% chance to grant Targeting Countermeasures reducing all damage taken by 1%. Stacks up to 5 times. In addition, Charge and Pull effects used against you have [25/50]% reduction in range.

It won't prevent Charge or Pull effects or try to turn Mercs into Snipers, but it will give a little bit of room for kiting, at the moment our range = charge / pull range so there is no ability to kite any class with either a Charge or Pull (or both).