One simple pool.

• Each henchman group entering combat adds 1 point to the pool
• Each non henchman adds 1 point to the pool
• Each Nemesis adds 3 points to the pool

• Each round the pool refreshes 1 point and one for each nemesis.

• Adding expertise/challenge costs 2 points
• Adding misfortune/fortune costs 1 point

Let's say you have four goblin henchman groups, a goblin shaman nemesis, four orcs and a troll.

Then you'd have a base pool of 11 points and a refresh of 2 points each round. That replaces all A/C/E and defensive actions unless you want a specific NPC to be more defensive.

Nice. Clean and simple. Just how I like it

I recently played the new(ish) 13th Age RPG, and that had a really nice mechanic called the "Escalation die". Basically, every round after the 1st, the players get a successive +1 on all skill checks (it's a d20-based game), so that the longer the fight goes on, the more the balance tips in favour of the PCs. I like the way that this reflects a lot of cinematic scraps, with the hero taking a pasting to begin with but hanging on to win in the end.

You could always dump ACE and do a similar thing for WHFRP. Give every (non-minion) NPC X fortune dice (to be used as desired for attack or defence) on the 1st round of combat. Then each round after that, strip one away until they're down to zero.

Yep that's one way of doing it. A set dice pool for each round and then remove a die from each type every round. That would work too. But one common pool is simple enough :-)