Honestly in this game the buster is so overpowered that that's not even a challenge imo, it would have to be without charging it for a true one.

i did the challenge in the power fighters

Do you have a video of the challenge?

There's actually a bit of routing involved in Buster Only, and really the most a buster can do is 6 damage, fully charged. You'd have to decide the best Robot Masters to take down first.

Wood Man in 1-2

Magnet Man and Plant Man in 3-6

Turbo Man, Junk Man, Shade Man and Slash Man in 7.

These are the most i-frame or movement heavy and it's best to knock em out early for Buster Only.