In this video, we are gonna cheese that Inferno guy with Cassandra, by spamming the same move over and over. It works. Most of the time. Yeah.
I casually discovered this after struggling to beat him. My time went from 76 minutes down to 20. Nice.

Timestamps of the steps:

0:00 Step 1: Pick Cassandra
0:15 Step 2: Spam Heaven's Guardian
0:34 Step 3: Reach Inferno
0:54 Step 4: Create some distance and spam the move
1:01 Step 5: Enjoy the video