Fool proof strategy: Everyone bum rushes Scanlan first turn. His AC is really, really bad, and Sam is a returning victor, so better not take any chances. Then, Percy and Vax get the hell out of dodge for the Kaiju battle between Keyleth and Grog, and wait until both of them are low enough to be OHKO’d by a sneak attack or a Percy action surge round. Though the latter has a very high failure rate. We then get the ultimate goth-off we deserve, likely to be won by Vax because if he has the option to hide, Percy is going to have a really, REALLY hard time of spotting him. Vax wins.

Other option: Keyleth zaps Grog anywhere with planeshift and no one forgets he has -2 on all his saves. Then turns dragon and goes off to roast some edgelords – with her wisdom, she has a chance of spotting Vax at any point and pay back some of that psychic damage their relationship has caused her.

(Though to be fair, Vax and Percy both have a decent chance of breaking Keyleth’s concentration. They’re also likely to have a higher initiative. Knocked out of dragon form, and at the business end of a Percy round with action surge, she’d go down and we get another edgelord-off. Vax wins again.)

(Also Percy is probably going splat in round two at the latest. And after giving the snitch away, he doesn’t even deserve any better.)