Counters are when you wear an enemy down until they’re blinking red. Then wait for them to attack and hit when there’s a red exclamation mark over their head. You’re better off doing this as much as possible in the first ten waves as things get a little hectic after that. This does have to be done during one session and is not cumulative. I found it was easy with a co-op partner. While one of us would be doing a counter move the other one would keep the other enemies attention and soften them up.