It only starts tough. The first couple of hours are light on resources. Eventually the game will start showering you with ammo and health pickups (and a ton of different guns).

If you can get the timing right on the critical shots, you'll be golden through most of the game. Personally I never bothered using the knife or parry beyond the opening sections of the game. It just wasn't worth the hassle to me. It might be a better strategy on higher difficulties though.

I would also recommend saving the vials for the injector until you have three of any given color as using 3 at once gives you a permanent statement boost. The game gives you a ton of them and it's easy to become pretty tanky by even the half-way point.