What sorcery is this? Three player Mercs?
We're not even a week in and the promise of mods continues to deliver.
I have watched a couple of his videos, they provide a great insight into how one should approach the different stages, but my biggest problem is pulling off those evasion moves and getting the timing right for the counter moves.
As said above, the counter window is mostly very small right before you're hit.
With some enemies it's fairly telegraphed and easy (Bloodshots, 2x4 Zombies) while for others it can feel much more awkward (Lunging Dogs and Zombies). These Mercs videos usually highlight a fair amount of countering, so they're actually pretty helpful in noting at exactly which point of the enemy's animation you should shoot to hit.
Evasion is much easier by comparison. It's mostly staying aware of where everyone is, what everyone is, and reading the enemy patterns.
Dogs, for example, are pretty simple to keep at bay with a little herding and lateral movement. You will notice that once they start running they become pretty committed to where they were facing. A minor step out of way can have them ending their run close to you, but they then remain stationary for a few seconds opening up a second dodge or quickshot.
Dives have an invincibility window that can be exploited. Dive back to open up some distance, dive to the sides to open up an escape.
If you choose to stay on the ground and shoot, make it quick and remember you can still roll to the sides from that position.
All in all, nothing that won't become more natural the more you play.