I've used the bloodied condition. I use this as both a descriptor of how far through an enemies hit points the players are as well as a trigger for reactions (though they can always do it, even if they've used their reaction).
I was trying to figure out minions, I think MCDM has a good way of doing them, but I haven't used them yet.
I have templates sort of based on the monster types in 4e, artillery, soldier, skirmishers, etc and I also have an elite template that turns a minion into a mini boss level character without going full legendary.