I started my millionth run of Reborn, to get the Reshiram path while also trying out new Pokemon strategies (exception being Pickup, Support Pachirisu (nuzzle to shut down fast Pokemon, charm to shut down OP physical attackers, I also used Super Fang to shut down bulky mons until this run) , I just can't give this Pokemon up).
Anyway, I noticed that Aggron gets both Sturdy and Metal Burst, so I slaughtered Goldeen and Seaking to make it max HP, max attack and defeat Arceus. Aggron as a Pokemon is already pretty strong, but without Mold Breaker or multi-hitting moves, there's actually nothing the opponents can do to stop a Metal Burst to the face.
I decided not to use it after the Arceus battle, mainly because I already used it as a main in another run, but also because it's way too good, but after getting swept by the fighting gym Meinshao, I grabbed it from the PC and put down that thing down in an instant. Having focus sash doesn't help either, since I can just setup some Stealth Rocks to make sure those hits kill. It's obviously not flawless, but it can be an easy bandaid for any difficulties you're having in the game.