Damage Up is the "big one" because of the prevalence of Defense Up as standard fare and as a dedicated .52 counter. That same effect carries to the Splash Wall's enemy damage to some extent (while not a constant presence, a .52 shot and a frame of Wall contact kills without Defense Up), and increases the killing power of both versions' specials.
As far as numbers on that, though, 2 main/4 sub or 3 main/1 sub is enough to keep 2HKO against nothing but Defense Ups.
Other useful abilities include Defense Up (for obvious reasons), Saver Sub (improves sustainability after Wall and especially Seeker use), and Tenacity (a faster charge without suffering from the .52's poor coverage).
As far as particular gear recommendations:
Any of the neutral-brand Damage Up items (Legendary Cap, Samurai Helmet, SQUID GIRL Tunic)
Bubble Rain Boots (Shoes, Damage Up, Inkline)
Armor Replica Helmet (Head, Tenacity, Cuttlegear)
Power Mask (Head, Defense Up, amiibo)
Black 8-bit Fishfry/Carnivore Tee (Clothes, Defense Up, Firefin)
Retro Sweat (Clothes, Defense Up, Squidforce)
White Tee (Clothes, Ink Saver Sub, Squidforce)
Blue Peaks Tee (Clothes, Ink Saver Sub, Inkline)
Octoling Armor (Clothes, Ink Saver Sub, Cuttlegear)
School Shoes (Shoes, Ink Saver Sub, amiibo)