A question we discussed lately:

Question: If the Genestealer swarm targeted by the “Power
Field” Action card merges with another swarm, may the
Genestealers be slain by other special abilities such as from the
“Overwatch” Action card? May the Genestealers in the swarm
be slain with the special ability of the “Door” Terrain card
when travelling?
Answer: No. All Genestealers in this swarm cannot be slain (by
any card or ability) for the remained of the game round.