Lvl 10 (Sir Steadfast):
Run around him in circles and hit from behind as many time as possible, after he tries to hit you you can attack from any angle.
Lvl 20 (Big Green One):
Stay above and hit from the top once, after hitting its eyes should flash, when it leaps don't run in a line, run left or right away from it, push when you see eyes flash to stop it leaping but while push is cooling down try and hit from above. If health gets low, kill the little green slimes for a high chance of a life dropping.
Lvl 30 (Lone Ranger):
Stay as close as possible, if you see him readying his bow let him fire an arrow and push it back at him, then get close again and slash the shit out of him. Possibly the easiest boss.
Lvl 40 (Mc Fisto):
Again, stay very close and try to get into the rhythm of when he hits you, every time he swings at you hit him, and just mash the attack button until your thumb is sore, instead of running away or around him run into him in a straight line and stay touching at all times. Just make sure to stay away from the edge.