On the subject of Oblivion, I imagine making him into something that isn't effectively a Skeletron Prime clone (no offense but that's basically what he is, and it's kinda boring) would help a lot with the imbalanced issue.
To be frank, almost every boss in Exxo Avalon, with the possible exception of Bacterium Prime, is pretty much a clone of some other boss or enemy. Granted, I realize you're not the best at coming up with boss ideas, but a lot of them aren't that... interesting. (NOTE: Desert Beak is an... okay boss. But that charge is way too fast and nearly impossible to dodge.)
Seriously, if you need help with boss ideas, you have my Steam information and can bug me on there. I am literally a walking idea generator.