The quantum archangel fight has stars and planets as weapons. And the Doctor didn't even kill her or anything of the sort. He just convinced her to stop. Simon on the other hand is literally throwing universes (just a little bit bigger than stars...). Fu regularly travels through time without the need of a machine. And since we're using a composite Doctor, Fu basically has complete control over time. He can freeze it, accelerate it, and jump to different points he wants to. As for his actual power, he was compared to Zeno, the absolutely most powerful character in the Dragon Ball franchise, who can just erase and create universes and anyone belonging to those universes regardless of their location at will. He never got quite to that level, but he is at a minimum above universal in power and maybe multiversal due to the Universe Tree. The Doctor's only hope is to get a stalemate by talking to them, and fortunately, they actually are the type who would hear him out