Chris chapter 2 has 3 ogroman, 2-4 sterlats, 8-12 napads. Steel beast in mercs has 6 napads, creature workshop in mercs has like 8 napads and ubvisto, rooftop mission in mercs has 6 napads and 2 ubvisto. so many places besides these to find the enemies for that event and chapter select being improved makes it easier.
I just go with chris chapter 5-1 in the room with the conveyor belts. Can get quite a few respawns if you leave the javo to pull the levers although I dont know if there is a more efficient way. Seems faster to do playing as piers as A.I takes a bit of time to help chris through the fans.