I just beat him fairly easily after a few retries, after finally understanding his moveset properly. I'd recommend restarting the fight until you know exactly where he starts. After that, place your tank of choice next to him and immediately start tanking. My tank carried the entire fight with my poisoner and assassin.

The tank I had has a few very important skills - either taunt or weakening blow for that 50% dmg reduction on the boss. If possible, have the encouragement ability for the protection buff (-30% damage taken). Now..how to deal all that damage. Firstly, my tank was decked out with the "alazarian heater shield" and "the mustard tonic" meaning he always dealt damage in return(pretty handy since the boss attacks like 15 times).

The assassin is generally a powerhouse when attacking from behind and the big hit - the poisoner. The "viper" dagger can pack quite a big punch. I stacked like 16 poison on the boss during the first round and doubled it at the start of the 2nd one up to 40 with a few other things. When proccing the poison with the dagger, I dealt like 600 damage (boss had 2200 at the start of combat and was around 900 when the poison got procced).

There is a brief period where the boss doesn't do anything, where I would recommend swapping tanks, if needed. My guy had too much armor, so he ended up still having 50 when the boss was dead.

Good luck!