Is Boo getting you with serves or the general slices? Either way, being closer to him makes it easier. When he serves just walk up to near half court, and you can easily return before it curves either way. Due to the way the counters work (return a charged topspin with a slice, return a slice with a topspin, flat with flat) you probably want to lean on flats more as Boo has to keep sending the same ball back or he'll spin out.
Waluigi and Bowser Jr. just have insane reach. They can basically stand in the middle of the court, start charging for a shot and hit either the left or the right barely moving AND still getting the charge shot off as they have way more leeway on how near the ball they need to be to hit it. They're literal walls that are very hard to get through. This also means they're efficient for meter building as you get more with charged shots going off. Chain Chomp is technically Power but is so fucking huge he's almost like a Defensive character just naturally because of his body size, but he has less leeway to be near a shot.
Boo is mostly picked because he's good against newer people, the same thing was happening with Rosalina in the demo just due to curving shots and most people don't really move up at the start. I think he's pretty mediocre personally.
