Make it only active if the player on which it's about to take effect if the alien hasn't been damaged by Marines in the past 3 seconds or something. If they take damage from falling, or a map entity, or a teammate - then it's fine. Make the 3-second timer reset only after damage caused by Marines.
Results: skulks take the same number of bullets to kill as b5 (helps Marines quite a bit), while higher lifeforms on the offensive can still easily stay on the offence, not needing healing as much from the hive/gorges.
Hmm?