The Perfect Carry named backpack has a Perfect Bloodsucker talent that grants 12% bonus armor for 10s (vs 10% on regular bloodsucker). This talent is already borderline useless on group heroics due to the "on kill" requirement, especially on missions where enemies spread out.
Instead of buffing the amount of bonus armor, increase the duration of that bonus armor - e.g. 12s instead of 10s.
Kills should count towards the talent, if you HIT the target, and the target dies.
So, i shoot the target, my friend finishes him of. I get the buff.
I believe, massive has already did that with Global event counting. They should do the same with on kill talents and Armor on kill. So if you hurt the target for at least 1%, it's death will be counted