The hardest angle of a Master BP is not giving them anything particularly great.  But if Mesprit can get Freevenge 9, I think Wallace can justifiably get Fuel Economy as a support for this.

Muddy Water is the big focus, being slightly inaccurate, but having a chance to debuff accuracy.  This functions as a complement to May, who...honestly has some trouble with the debuffing.  She’s still good, but there’s always room for improvement.  It’s also nice to just have an alternative for accuracy debuffing, given Hapu’s EX needs the spread, and there’s always SS Lusamine memes.

Calm Mind is the easy +1 Sp Atk/Sp Def, but with Stat Share 9, guarantees it goes to everyone.  It’s incredibly slow-ramping, so bad for CS, so I’d like to see a 3/5 skill for doubling up.

Heal Pulse is a quick pseudo-Potion, with better ability, but no MP recovery outside of one on sync via 3/5 grid.  Nothing major.

Trainer Move gives +4 speed and gradual healing, which is really nice for Fini, who can buff using an attack that kicks off healing.  Free Moves Next on the team is a bit awkward, but you know.  Do what you can.

I’d like to see First Aid 4 and Team Sharp Entry as 3/5 skills.  Team Sharp Entry isn’t huge, but it works well for May to not need MPR, and for SS Lusamine if going for the memes.  Quick Cure, Unbending, and Stalwart would be the 2/5 options to prevent defensive debuffs and remove an instance of Trap or Flinch.

It’s not exceptional, but I think the accuracy debuffing would be a fairly unique niche that works well for older and F2P units that need this kind of support.  Slow ramping Calm Mind buffs can be incredibly good with some healing in the back against Gauntlet fights like Latios, Latias, Moltres, and Tornadus, who are largely special.  The speed buffs are just great in general for Gauntlet.  I think he’d do okay, but not like...exceptionally well?