Great Game, and heres some tips to anyone struggling.
1: Guns are important, the more you have, the less you have to rewind, and that means you can use your rewind for more important things, like...
2: Rewind is MUCH more important than reloading. Your ability to regenerate lost health is how you win. There is an upgrade that makes it to where when you rewind, you get +4 health, this is important. As now, you can do what I call, "Rewind Spam"
3: For those of you wondering, rewind spam is when you spam rewind, regenerating your health back to full, this is best done with 1 enemy left on the field. Heres some of the best enemys to do it to:
Bat, Pistol, Gernade, and while its difficult, you can do it to drone. The others? Well, you'll probably get hit and kamikazi, if he does hit you, will just start the next wave.
4: Prioritize Health over damage, it sounds stupid, but the more you can heal, or not take damage, will do much more than kill an enemy faster, but this doesn't mean never go damage, for later rounds damage is important, but early game go for healing and damage immunity.
Those are ones I came up with in my time of playing, but practice does make perfect, so using these will take practice.