The thing with this game is that it heavily suggests (heck, it almost requires) that you learn to adapt and improve as a Pokémon player. If you use the same methods here that you use in a casual playthrough, you're going to have a very rough time. That's when you start thinking, "Well, my usual strategy isn't working, but what if I try something else that requires more forethought or understanding of the game mechanics?"
For instance, I beat Arceus back when it was still Level 80 by hitting it with Toxic until El ran out of Full Heals, then eating up all of its Focus Blasts with Blissey. It turns out that a Level 48 Blissey takes a little under 66% from a Focus Blast from a Level 80 Arceus, which means it can set up a Light Screen and start outracing Focus Blast damage with Soft-Boiled heals.
(It is also worth noting that I do not use any Potions, Revives, etc. in my Reborn run. I restrict myself only to hold items.)