there are two options for two different scenarios. if you've played through the base game and haven't play the DLC then you should use Sylveon with Amulet Coin as it's item, Pixilate as it's ability and Hype Voice as it's move
Sylveon
Level 100
item: Amulet Coin
nature: Timid
Ability: Pixilate
EVs 252 Sp Atk 252 Speed 4 Def
-Hyper Voice
you only need Hyper voice for this Sylveon setup as you'll be spamming it all the way! you are going to need a turbo controller if you don't like mashing "A" all day though
if you have completed the DlC and played through both the "Teal Mask' and the "Indigo Disk the the entire Ace Academy Tournament will have changed so unfortunately you're going to need something a lot stronger in the form of White-Kyerum!
White-Kyerum
item: Metronome
nature: Timid
ability: Turboblaze
EVs 252 Sp Atk/252 Spd
-Ice Beam
same with Sylveon just spam A and Ice Beam will get stronger with each hit thanks to Metronome
WARNING this set is not guaranteed to work I am still in the process of testing this, all the Youtubers who make videos on this set do not account for Clavell as his team uses Trick Room and he often wins. I will get back to you when I am done testing, I'll comment on this with the Updated method when i'm done