Set it to story mode just for that boss, thats what i did.

Lol. get gud? No really though this boss is the most fun engaging thing the game has. Yesterday did an act 1 race with my friend beat him by a few mins, today we did act2. I died to act 2 boss 2 times but very rewarding kill on third try. And mind you this is with very underleveld, under geared, under skilled characters because we didn't spend time at all to work on that stuff just went asap to the boss to finish before each other. I did it with a caster which was added hardmode. attack speed so freaking slow to regen willpower hard to dodge stuff and still do dmg. really fun fight I wouldn't want it changed.