It feels too joyful for a boss battle. All major chords perhaps, but it doesn't feel that intense. It sounds nice, but I don't think it would fit a boss battle.
but a MINI boss battle... eh ;) Ima put it this way, I want my game to have as much contrast and variety as possible so I don't want every boss battle including every mini boss battle to be serious. I think its more interesting as a whole and I think it highlights the contrast in tone in major boss battles very well. I wouldn't have denied this idea if I haven't already thought it out but I don't think its a false prospective either.
Please contact me if you would like to use this in a project. We can discuss the details.