I had the same issue. He took my whole HP in 3 or 4 hits. I tried with 3 different companions decently equipped (healer, tank, dps) but didn't work.
Same thing happened to me with the final bosss in Makeb. It seems that these bosses become bugged if you are in a party and become impossible to kill. I managed to kill that one though by relogging, but that doesn't seem to work with this other one.
I asked a friend to help me kill it but it's also proving extremely difficult... right now this quest looks quite impossible for me.