I see a few problems with Thundergod Kutum.  And some good points.

Before I begin I will say that I tried multiple characters with Oasis gear, the only ones that seem to be able to survive is wizard/witch because they have all the neccessary skills to survive.  Those skills are, Self heal, high mobility (to clear debuff), good access to FG abilities, ranged, and the big one able to cancel any skill with a PvE I-Frame (magical Evasion).  It seems that every one of the new bosses the best character to use is a witch or wizard, they upgraded bosses need to be made more melee friendly and not just witchard friendly.

First, the "dance" you do at the boss is actually rather good, I would however remove the 75% & 25% Super nuke it does, there is no reason any boss should have a nuke that removes 4k health of a player in Oasis gear with no actual targeting notification.

The baby Kutums have way too much health to take down when Thundergod doesn't spawn at prime time.

The DOT Debuff is not noticeable enough, I actually didn't notice I had it on me many times on the witch until it got down to 20 seconds a couple of times.

The Debuff cleansing giving the "big buff" doesn't happen often enough, and the "big buff" should be far more powerful.  An attack speed buff just isn't worth it for how rare it is.  Either make the buff every time or make it so that you get a huge bonus like 300 monster damage for the 10 seconds.

The random damage where no red circles were showing needs to be fixed, many many times you take damage or have 60% of your FG just disappear when you are not in any one of the red circles.  It should be possible to do the boss with no armor on if you do the dance perfectly, but that is not the case.

There needs to be an incentive to actually hit the boss more than just 1 time then go AFK for 29 minutes until Kutum Despawns.  I would suggest adding more baby Kutums wiht less health, less armor, and they spawn in the tunnels/chambers at super strength until they actually reach the chamber where they are "weakened", once killed they should drop a gold bar or 2, and a item that if you bring it to the northern chamber you weaken Kutum allowing him to take far more damage.   The reason I say make the baby kutums spawn in the tunnels is so thaty the can "clear out" the tunnels and chambers of AFK players, make them move slowly but do a huge amount of damage.

Just going to reitterate one point, the upgraded bosses need to be tested in Oasis gear on all classes not Pen C9 gear.