Strategy-based game despite the huge luck component.
Yes what the enemies will do does seem random, so mostly mixing it up with what you do yields good results at least eventually.
But what you really want to do is look at the arm stats and equip the better arms as that'll really make all the difference and don't assume later boss equals better arm because it doesn't always and some boss arms are better than several bosses beyond it.
You want large offense and also large KO threshold stats and pay attention to which attacks you get the offensive bonuses on because you'll want to focus on mostly using those attacks and hoping the enemy doesn't counter that strategy.
Early on you can just go with equipping the next arm, but sometime around Paleozoic you should realize "this arm is terrible". You'll probably need Purple Nurple before you can go back and get Hambone and then Bulldozer and of those you should stick with Hambone when you get it.
Once you get Bubble Boo pretty much just stick with that arm for a long time though you should switch to Bear Paw once you finally get there. Once you finally get Grave Digger, stick with that for the rest of the game and/or swap it out with FlySwatter which may be needed along with luck to get some of the remaining arms.
Most of the other arms are either unremarkable or bad, with the exception of Royal Flush which is pretty good, just not as good as Grave Digger or FlySwatter which you will probably have by that point anyway.