Tbh I'm stumped. The reapers need to stay long range, but no matter what mechanic you try and use (execution cooldowns, majority damage requirement) long range executions are going to lead to execution poaching and or farming. Plus it will be hard to make them cool. The one option that does appeal to me is maybe having a limited supply of "special" ammo that negates the downed state. A bit like how the lascannon currently vapourises people. Obviously you wouldn't want this to be happening all the time, but having the option to make sure a high threat target doesn't get back up would help compensate the reapers for not having a execution option. I would emphasise that this option should be very limited though and not consider as a excution. Negating the downed state from a low risk long range posistion will heavily affect the balance of a firefight.