Bashful is a neutral nature, so just go with the normal stuff. I would recommend attack, HP, and defense.
Good EVs for Choice Band Swampert: 172 HP, 252 Atk, 84 Spd.
Good EVs for Physical Tank Swampert: 240 HP, 16 Atk, 252 Def.
Source: Smogon UU and Smogon OU
