Dragon Fogel just taught me how to beat a boss by traveling back in time to 2008 and

You're pretty much restricted to squatting and bumble-canceling against Hardened Basin if you insist on not using the hammer drop (i actually had to do it this way my first time playing because i didn't even think of using hammer drop lol)... during spin mode you have a 3 frame window to perform the bumble cancel without the mini gun rays hitting you, if done correctly it will reset it's attack pattern and you can go back and reposition yourself. I don't know why you'd commit to a bumble rocket only run though since it's almost entirely dependent on RNG.

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(Hard mode LVL 1 only is an actual mode on Castlevania OoE)