Strafe, jump, run away. I don’t know if that’s particularly helpful, but you’re not wrong about the difficulty.
Early on, the flare gun is your friend. Monks and zombies only need one shot, and the flares have a fairly long range. They do take time to burn though so you might need a corner to lurk around for full effect.
Dynamite is wonderful on groups. Don’t forget alt-fire so you can bounce it around corners.
This game is notoriously hard. But also there’s a bug which causes the monsters to do more and more damages as you save & load. The bug is present on all difficulty levels except for the default one. So I guess playing on the default difficulty is the only option, unless you don’t want to use saving & loading at all.