No, you don't have to fight them in the BP but that part of the game is scripted for when you enter that certain area.

Are you sure you don't remember which level such an area is scripted? Eh, I probably should just play through the rest of the game at some point to see what you're talking about.

I don't remember what level number but you ended up near a pyramid or castle of some sort and the playing stop anda cutscene appears. It shows you the Black Pyros and yourself than goes back to the gameplay and youi must kill 3 or 4 of them.

OK. So is the fourth level of Retribution not compatible for co-op because you fight PTMC mercenaries?

As far as I know the entire Mercs expansion wans't coded at all for co-op. So if you would want it to be, you would have a huge undertaking to get it to work. It w as made for 1 person only.

Isn't level 4 not compatible for co-op though of Retribution? Or is it? I'm asking this about D3 Retribution, not Mercenary.

I've played D3 level 4 (subways level) co-op. The original D3 is co-op playable. But the expansion isn't.

Well, I've played through the entire Mercenary now. I didn't fight any Black Pyros. That only happens in Retribution. I also see a FEW ways in which the game was designed for single player instead of co-op. But besides level 3, only a few small modifications need to be made to make a Mercenary co-op mod. Sure, there's goals to do that only ONE player at a time is going to do, but this happens in Retribution and all KINDS of co-op modes in various games allow only ONE player at a time to make an event happen.

I say a Mercenary co-op mod is completely possible. But it would take a lot of work. I may do it, but likely I won't.

I will have to play D3 Mercs again because I swear the Black Pyro fight was in that one. The Black Pyro was intriduced in Mercs and you couldn't play D3 multi with it until it came out.