Phantom and i would be using the much much more effective method of cloaking until melee range than synckill. If using standard synckill rules, then cloaking until melee range, melee once, synckill, melee again, synckill again (for ops packers resisting synckill).
Honorable mentions to
i) Collector captain. Hide and ambush with super RoF CSMG. Everything less than krogan health/shields less dies immediately even from full shields
ii) Reload cancelling rocket trooper. With this guy, almost everyone dies from triple+ shots, from range. Just make sure you're shooting them in the back so they can't dodge. Against juggernauts and those vets skilled in dodging based on sound cues, don't stop reload cancelling until the target dies from infinite missiles.