Calm Mind would be more useful than withdraw... so replace Withdraw with calm mind, and as a last move go with Scald, Mega Slowbro is a solid wall... so a move that gives status would be very effective. Especially with calm mind
Also for EVs go with 252 SpD/ 252 HP/ 4 SpA
So the set will look like this
Slowbro@Slowbronite
Ability: Oblivious
Bold Nature
EVs 252 SpD/252 HP/4 SpA
-Calm Mind
-Slack Off
-Psychic
-Scald