You have Quint left to fight.
You'll need to climb the waterfall in the hub after beating the MM9 final boss.
Quint is a very difficult and unforgiving fight, even by this game's usual standards, so I won't blame you end up cheating through it.
Also, there's the Wily Capsule, which you can fight after beating Bass' second form without using the Super Adaptor.
Ah, thank you! I have now just beat the wily capsule, and it is not as bad as I thought it would be...
Although ough quint was a pain. I like the "Time Travel" stuff, but it could have been done better.
In conclusion, the bosses need a lot of nerfing.