Tyraniking, I know sword Level 2 is more or less a boss killer. Spamming that in front of any boss has given me good results. Level 3 is also good but it's slow, rapid firing the second level of the sword is still strong but it's much faster, especially if you have a good trigger finger. I know the speedrun did that boss the same way, level 2 sword and 8 hearts. Got up close and started spamming away. Of course I can't rival a speedrunners tactic so I probably should go for something more...subtle.