I think it basically depends on the health of the said enemy. I think the only enemy that we can break a sync-kill fast enough would be the Banshee and a Praetorian...A Brute, Scion, Atlas and even the Phantom sync-kills are just too fast to try and break. I will say that very rarely i have saved a few teammates from phantoms right as she was raising her sword, i would hit her with a B/C to break the animation...it doesn't happen all the time tho...
For the most part if i see a teammate about to get synced, i will whip out the rocket, because that will guarantee a save....other than that i will try to hit them with B/C (yeah i use the phoenix ALOT...
) or i will hit them with Smash (which actually works half the time...
)