As one of the first who got Surt and has been able to fly him on many bases, maxed or lower, he isn’t that broken at all. If people just line up 2 red mages on a given island, he is powerless and will die very quickly.
If you do insist on nerfing him, which I don’t feel is necessary, simply adding 1 second cooldown to Burning Hatred will penalize him enough, giving more time for all towers to hit Surt. Or play around with the notion. Messing with the combo Incinerate+Ragnarok would just break the dragon completely.
I honestly was applauding the fact that he was really balanced, just forced people to rethink their bases, which is a healthy concept for me.