The way I got this post game was to fast travel to the supply point in The Vision called "The Vision High Rise" it's also the only fast travel in this area. I initially tried this on the enemies that always spawn on the highway but lasers were attacking them there. If you're fast you can get it there. The spot I got this was on the same tower as the supply point I went 2 levels down and came in from that highway side I mentioned earlier. If you miss it you'll have to just reload your game and they'll be back. My game is 100% complete all locations.
Edit: top of the science center in central shade has all the enemies you need. It's actually better than the original location I mentioned. The very top has nothing but small enemies spawn on two sides using pea shooters. Just hang out, let a bunch spawn in, and do whatever achievements you need.