Since I plan on speedrunning this in the near future, I was wondering what is the best order to defeat the mages?

It depends on the category you run and also your deck. You need to save in between if you can't be sure to actually beat the high mages. If you are unsure of beating them you can also decide if you want to do easy mages first like ocean or mountain or hard mages like meadow or forest.

If we would be talking of New Game+ or any other scenario in wich you wouldn't save between the mages then the fastest order would be ocean -> mountain -> meadow -> forest -> desert