Finished it with:
Lv 100 Garchomp (Double Edge, Earthquake, Outrage, Iron Head)
Lv 100 Charizard (Fly, Flamethrower, Thunderpunch, Outrage)
Lv 87 Venusaur (Frenzy Plant, Sleep Powder, Leech Seed, Sludge Bomb)
Lv 85 Throh (Superpower, Reversal, Bulk Up, Stone Edge)
Lv 75 Thundurus (Thunder, Psychic, Brick Break, Crunch)
Lv 61 Walrein (w/ exp share, I was trying to grind it while battling the elite4)
my team is weak lol. it took so much time to finish the leaaaaaague.