First, make sure your bag has a bunch of X speeds, full restores, and Leppa berries. Give this moveset to your Torterra.
Torterra @ metronome
Level: 59
Experience: 211,059
EVs: 252 Atk / 252 Spe
- earthquake
- razor leaf
- stone edge
- swords dance
This experience number makes Torterra reach level 60 after gaining one point. Stone edge covers stuff that resists both grass and ground, like bug and flying. If you have even more time, then you can get a lucky egg and a Torterra with perfect IVs, adamant nature, and boosted experience, but for this answer it's probably better to assume it has 16 IVs and a neutral nature.
Save right before fighting each person so you can easily soft reset when stone edge misses.
Aaron: His fastest Pokemon is a Drapion with 130 speed, so you need one X speed to outspeed the entire team. You need 2 swords dances to OHKO his entire team, partly because Heracross resists both STABs and is not weak to rock. After this battle, Torterra should have 211,693 experience, which puts it at level 60.
Bertha: Her fastest Pokemon is Whiscash with 87 speed, so you already outspeed her entire team. You need 2 swords dances to OHKO her entire team. After this battle, Torterra should have 231,413 experience, which puts it at level 61.
Flint: His fastest Pokemon is Infernape with 170 speed, so you need one X speed to outspeed his entire team. You need 1 swords dance to OHKO his entire team. After this battle, Torterra should have 243630 experience, which puts it at level 62.
Lucian: His fastest Pokemon is Alakazam with 167 speed, so you need one X speed to outspeed his entire team. Unfortunately, none of Torterra's moves can OHKO Bronzong, even at +6 physical attack. His Bronzong doesn't know any particularly dangerous moves, so you should have enough turns to use more swords dances and stone edges. After this battle, Torterra should have 254,126 experience, which puts it at level 63.
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