Traits that trigger on skull damage (Bullseye, Assassinate, Psion’s mana steal, Chill Touch, Aflame…) work differently from each other when the troop is entangled. Reducing their attack to 0 might cause the same inconsistency but I haven’t checked.
Assassinate and Bullseye can trigger even when just matching 3 skulls and normally doing 0 damage.
I think Aflame, Chill Touch, Death Touch, etc. can trigger if you match enough skulls to do damage (ie 4+) but they don’t trigger on 3 skull matches.
Psion’s mana steal doesn’t trigger even if you match 4+ skulls and do damage.
I think there should be consistency across these traits. IMO it would make the most sense to have the traits trigger when enough skulls are matched to deal damage and make the troop do its attack animation. At the very least poor Psion could have his ability treated the same as the generic “on skull damage” traits.