I'm pretty sure a Fallen Captain finished him off for me during a Defend the Warsat. I was solo, and able to get his shield down and a few rounds in him...but then I died. Urzok turned & started going back and forth with said Captain. Next thing I know...I respawn & the event ended. I did not kill him myself...but I got credit for it. For what it's worth...I had a group with me on my other 2 characters...this was the only time I was successful solo against him lol.