(all after eliminating capt Merc first- watch HIS bombs too, though has no ground poisoning)
1. V swarm
2. sometimes full fatal burst breaks his shield
3. run from his bombs as they land and leave the space of slightly smaller than swarm radius
4. when you get him in a clear patch, wail on him with melee.
Dr.'s strategy.

(In co-op, I usually try to use within 5 packs.... but I have been able to co-op with 0-2 packs used)