I've used that exact Gallade before; it definitely does well enough to bring along. Psycho Cut is far better than Aerial Ace. :P
Night Slash hits the same things as Shadow Claw/Shadow Sneak and Shadow Claw/Shadow Sneak are leagues better than Night Slash on Aegislash. Also, without King's Shield or Shadow Sneak, Aegislash is unable to Effectively KO more than one Pokémon due to it's abyssal 60 Base Speed. Also, Life Orb makes this even worse as it takes HP away from Aegislash. I suggest you either give Aegislash Leftovers, Shadow Claw, and King's Shield; or Shadow Sneak. :P
Big Root+Aqua Ring doesn't heal as much as Leftovers+Aqua Ring. Leftovers is a better Item for Wishiwashi. While Wishiwashi isn't seen as a great Pokémon, I think it's bulk will be fine for the Battle Tree. Use U-Turn, Brine isn't a good move in the first place, and Pivoting could be nice. You might also want to consider Toxic to Ruin things you can't hit as hard. :P