Are we talking about them canonically? I'm not seeing anything in the Pokedex entries that suggest that his past form is stronger.
Hisuian Samurott:
>Hard of heart and deft of blade, this rare form of Samurott is a product of the Pokémon's evolution in the region of Hisui. Its turbulent blows crash into foes like ceaseless pounding waves.
Regular Samurott:
>In the time it takes a foe to blink, it can draw and sheathe the seamitars attached to its front legs.
>One swing of the sword incorporated in its armor can fell an opponent. A simple glare from one of them quiets everybody.
If anything, it sounds like the regular form is stronger, at least according to Pokedex. It oneshots any foe, outspeeds any enemy, and has Haki/Spiritual Pressure/whatever that glare thing is.
