I feel similarly about Sathanas, even in his 'weakened' form. His stats are completely bonkers, and you don't know whether he's going to use Furorem (Fire) or attack (Dark). Since you can't cast Radiance mid-battle, you basically just have to hope you guess right with which one you pick, since it's random. And since his agility is guaranteed to be higher than yours, even if you knew the pattern and could switch, you'd need a down turn to do it. I have seen that happen once when he re-casts Superbia, but usually it goes Furorem -> exhausted attacking, which I can't counter at all, so I re-roll for starting on attacking, and the number of attacks before switching to Furorem is inconsistent and usually doesn't involve re-casting Superbia. I guess I could play tactical advantage roulette, but that seems incredibly unlikely to work. If there was a way for me to go first I could use a combination of Pacify and the Humanitas Rune, but as it is I can only use one of them, I need the damage reduction from Guard too much on subsequent turns.
EDIT: Well, I just had a sequence where he went 4x attack -> Ira -> Superbia, and I wasn't prepared for that so even though I cast Waterskin after the 4th attack (based on having seen a similar sequence once before), I didn't realize I should also cast Pacify the next turn. RIP. Oh, and then it turns out he stuck with the Dark attacks.
EDIT: This time the Flash Bomb did work. I basically re-rolled until I got the 4x attack sequence, guarded through that, then Flash Bombed before the next sequence of attacks. I actually hadn't noticed before that Flash Bombs have priority. The sequence kind of went off the rails, as he did Furorem 1 and I activated Water Skin hoping to dodge against Furorem 2 and then he did a dark attack instead, but fortunately the avoid saved me.
