yes EVENT_HP will do this quite well. some points to note:
1) the spawned mobs will need to be tuned for aggro. if they spawn and are green to the players they might just stand there and watch their overlord get spanked. (set aggro appropriately similar to how undead ignore level for aggro).
2) add in a "despawn" timer or "upper minion limit" so that if the players attempt the mob several times, they dont end up having to fight several waves of the same minions.
3) add in some form of quest::shout or quest::say so that the players get some clue as to whats happened, otherwise they might assume the server screwed them with a spurious spawn.
have fun
== sfisque