Rather than relying on Trigger-once, which only works under certain circumstances, manage the logic yourself. Get rid of the first Wait, so that the bullet & collisions are disabled immediately, then replace Trigger-once with "Follownemy_hp.Collisions enabled". Since you've explicitly disabled collisions, you know that condition won't be met the next time round.
I also avoid Wait, so use a timer on the Follownemy_hp and set it to expire in 0.5s. Then do the destroy and Add in the "On timer" event.
The problem with Trigger-once and Wait is they aren't object specific. By making all of you logic object specific, you have 100% control over the behavior/logic of each object.