If you plan on getting the Go Get Some Fresh Air achievement, which requires getting 100% on all levels, you must complete every level by killing all enemies, finding all secrets, and never dying, difficulty don't matter.

From that point of view, one can say that enemies somehow are like collectibles. You'll get the achievement Mopping Up if you killed all of them in a level. But honestly getting this in most of the levels are pretty easy, especially if you also go for a hunt for the secrets and ores since those areas spawn extra enemies sometimes.

However, there's a level I played 2 times through, hunting all secrets, and still miss some enemies. That level is Meltdown, and here I will explain which ones are easily missed.

This stage has 2 lockouts, one in the beginning of the level, and another just before the end.
The first lockout happens when you enter the cooling tower, the room behind the forcefield in the above image. Make sure you have 20 kills before entering the room as you will not be able to go outside again.

Inside your path splits into 2, the green key path (left) or the yellow key path (right). The final lockout happens when you activate both controls for the cooling tower. There should be 124 kills before pressing the second control.

If you're going for all kills, as I suggested above you'd also better be hunting for secrets and ores. I won't go into detail for this but I will link a guide below that covers it. Be sure to thoroughly check the secret areas. As for the spawns, they both happen on the green key path, left from the room entrance.

The first one I dub the "Minigun Monolith". It's on the left side of the green key path, near the house with the opening on the roof you can enter for a secret. You can use that button to lower the platform or jump to the minigun from the building's roof, it will spawn enemies. I missed it because I didn't need the bullet ammo so I just kinda ignored it.

At the end of the green key path, you will open the forcefields for these moving acid vats you can ride back to the main room. Instead of going back the way you came on foot, ride one of these things. Some flying enemies will spawn, if you go back any other way they won't spawn.

Big thanks to DemonEyesKyo for creating a guide for the secrets and ores. You can view it here: https://steamproxy.net/sharedfiles/filedetails/?id=2890084001
And I hope my guide helps you in completing all kills for the Meltdown level. Please let me know if I got some kill numbers wrong or if there are any more you consider easily missable spawns in this level.