You should be able to fly through mooshu at 55 but lets take a look at why this issue is happening. Can you give us some details; What school are you? Whats in you deck (roughly)? Health?
Well, I would suggest you upgrade your gear and maybe a higher resist. What may also be is that you underestimated your enemy and discarded traps/ blades/ shields? Or the enemy might've been the same school as you- you coloud've also not watched your pips. And how are you coping in Celestia? If you are doing great then I think one of the above is the reason you couldn't defeat a moosu boss.