Just my two cents, but I wouldn't really like it if enemies were essentially unkillable for a few seconds, since if the enemy is an Energy Leech Eximus (Butchers, crawlers, runners as some examples of parazon-able enemies) or some other type that I would like dead yesterday it would mean they would have a random chance of being unkillable unless I get up close and personal. In that time Energy Leeches Eximi can take 15 or so energy, other types of Eximi can continue to buff their allies (Toxic eximi as an example) or nullifiers could even have their bubble come back if you didn't destroy the drone for it. Unless the enemy is considered dead (I.e auras getting cancelled, bubbles being removed etc.) so that they're not buffing their allies in those invulnerable seconds I cannot see the suggested system not getting annoying when it starts messing with players, like the "Hold E to Heavy Attack" issues did. Edit: Had a random thought, an option could be to make it opt-in via the mercy parazon mods. If you equip any mercy mod to your parazon enemies who get a mark could go down for a few seconds, but without a mercy mod equipped they wouldn't. How's that sound?