Centaur War Beast bug
I seem to recall that thing used to sit around invulnerable for a while before its event actually began, at which point you’d be able to kill it. Are you saying the event just never starts now, or did you find it lounging about and try to kill it?
There are a bunch of other things you can do to support the troops. You can bring the care packages to the npc and train(fight) with some of the troops. That is actually the fastest way to finish that heart with a higher level(or equal depending on how confident you feel) character. Start a fight with all the NPCs at once.