Hi i've seen a lot of people asking for help but unable to find people online so here's a video to see how to beat the game alone (in bloody mode because why not).

Jason is the best with his 200% HP and his melee weapon letting you attack enemies and particularly bosses without the need to be away from them to attack.
Using his special ability just after an attack let you do 2 attacks almost at the same time.

A map with walls can avoid you getting pushed away from the map.

The real difficulties of this game are the glitches let you go through the floor or walls making you die instantly and the wave 6 boss because of his stun dealing you a lot of damage inpossible to dodge.

To avoid glitches avoid being near walls or little gaps when enemies are pushing you this will avoid some annoying insta death.
The best way is avoiding maps with holes like the first map because you can sometimes get stuck in the little holes.

You have to stay near the racks so when you are about to die you can heal up and keep attacking him.

This boss only need you to keep attacking him with a little distance from him and run away from him when he start running.