I'm just doing the mission "Coliseum" with my level 45 Fed-Romulan and noticed the arena battles have changed. There was a Mature Mugato in as the third opponent that wasn't there before. I found this battle comes down to whoever gets lucky with damage rolls.
It's damage capability was roughly equal to mine, and with no buffs or heals its all luck who kills who. And it fully healed every time I respawned. After several tries, I got lucky and it ran out of hitpoints before I did.
I'm assuming that "respawn until you get lucky" is not the intended strategy for completing this part of the mission. To that end, I would suggest enabling the healing plants that appear in the sandworm fight in the earlier fights as well.