Well, that was way easier than I expected.
1st round, I popped the Aether Cluster and DPSed him. At 12th second mark he was at 50% HP.
Then I started to kite. The key thing to remember is that it’s those damn flames that are deadly (getting in the middle of one would reduce my HP by half! from 18k to 9k lol), not Kymon himself. The flames are actually following a set pattern and are fairly easy to dodge, it goes like this: outer circle -> inner circle -> left side -> right side -> “cross” and then it repeats. All you have to do is to remember this while kiting.
My original idea was to kite him until my aether cluster is available again, but when I was about to attack him, he died. Lol! Guess the damage from blade spirits + Rattosh + revenant skeletons was actually big enough to shave his remaining 50% HP during my kiting.
Kymon would hit me occasionally by his jump (annoying during the "outer’ and “cross” phases as you need to run around close to the center and he can jump through the gap in the middle to surprise you) but he didn’t exceed 50% of my HP in damage.
Off to kill korvaak, thanks for all the tips.