I don't care what anyone says phoenix doesn't get tired she doesn't run out of energy llike galactus who needs to feed to even stand a chance and she has beaten him despite not even using the all of her phoenix force.
She is a matter of life and death so it is very important that she stays alive,the watcher said that she is the most powerful mutant in the world and with jean grey powers she is even more powerful.
Thor is not actually stronger than galacuts so he has no chance whatsoever against an entity as strong and resourceful as phoenix.