Thanks!
As for the difficulty my advice is:
1. Keep moving. The game is designed, so that baddies aim not at you, but a point a bit behind you, that moves toward you. If you move in relation to them they'll miss.
2. Try to keep the baddies at a distance if possible.
3. You can try to move back to previously cleared rooms and wait for the bad guys there.
4. Remember about medpacks and ammo! They respawn, so don't worry about collecting them all.