EV training normally isn't desirable in a standard playthrough, as it's much more worthwhile and less time-consuming to simply overlevel your Pokémon. In a nuzlocke or any other challenge run that outright restricts this, EV training is still rather tedious, and is only desirable for a long-standing team member, such as your Samurott. However, for a challenge run that requires your Pokémon to be at their best, it is worthwhile to do.
Samurott should prioritize it's Sp. Atk EVs over Attack. Not only is its Sp. Atk stat slightly higher, but its Special movepool of Surf, Ice Beam, and Grass Knot is generally superior to that of Razor Shell / Aqua Tail, X-Scissor / Megahorn, Dig, and Superpower. Swords Dance either requires significant grinding in the Pokémon World Tournament, or waiting until the E4 for it. The only deficit to doing this is Dewott doesn't benefit from getting Razor Shell immediately upon having evolved as much as it would with Attack EVs, but it should be compensated for an easier lategame.
As your opponents won't be having EVs themselves, Samurott should be able to outspeed the majority of opponents will full investment in Speed. If your nature is one that lowers Speed, it's likely more worthwhile to invest in HP instead. If you're willing to EV train however, it's probably worthwhile to reset for a good nature, assuming you aren't too late into the run. One that benefits Sp. Atk or Speed and lowers either Defense stats is best for Samurott. The rest of the EVs can go into the Attack stat.
At level 59, the same level as the Champion's ace Pokémon, Samurott maximizes its stats with 8 Attack, 248 Sp. Atk, and 252 Speed EVs.
