It’s planned to make each mode more unique, and we will continue to tweak the stuff that’s currently in. However, it is worth acknowledging that as you kill more enemies in a level, they will become less compliant and more resistant. This is intentional behavior, but at there isn't a point where they will not give up by any means. It just means instead of being able to yell you'll have to stun. The difficulty curve needs to move up if you're playing in such a fashion, that's our view right now at least.