We were in the arena for over an hour, a tanker and some other guys kept spawning, but we couldn't defeat Dekuso, his shield never went down and we did no damage to him.

We tried various weapons (even released the turkeys ^^), grenades and classes.

Tried leaving the instance and returning.

Upon group death, we still couldn't leave the arena. Until I dc'd and it ended in defeat for the others.

The others in my group tried again and it worked. So it is possible this was just a once off bug.
I am just putting it on forum in case others are having the same issue.

Thank you for your time

P.s. On the positive side the bug gave us kick *** scores *winx ^^