Mob becoming invulnerable in Temple of the sun map

Did a tier 15 map (Temple of the sun) and the mob (sorry don't know the name) that goes on all fours and starts shooting bolts at you became invulnerable and kept chasing me around and shooting bolts. Was 5 minutes+, tried zoning in and out again and changing the floors on the map but it didn't help.
Sadly it was one of the rare mobs I had to kill and couldn't finish the map.
Didn't take any AoE damage so nothing really worked.

Thank you!

got same bug.map cannot be completed

Got the same bug today, still in the map but cannot be completed : mob takes no damage.