Play on hardest difficulty, disable slow bullets, and go with the default weapons for every mission - it should make it significantly more challenging.
Random_box, the second boss is not that hard. If you're equipped with a weapon other than your fists, you should be able to kill it within 8 stuns. I killed it with the glowing red thing in 5 stuns. I just kept focusing on keeping my weapon in the field of play and sidestepping the charges.
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