DOTs do full damage which is how I did 54(?) and 58. Once you do a certain amount of damage (maybe 20k?) he starts taking normal damage and as long as you can keep damage up consistently he wont put his shield back up.
So I did DF with poison from hermit as my sub ability and used Cerberus' burn for 7333 per turn through shield. 4th turn shield fell off for me. Trying other comps with weaker poison / burn meant shield stayed up much longer.
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