To start off, Future Sight is simply not a good move. It has good base power, yes, but your opponent can simply switch out into a Pokemon that can take it well, such as Chansey, Dark, or Steel types, for example.
Now, here are two good Mega-Slowbro sets.
Slowbro @ Slowbronite
Ability: Regenerator
EVs: 252 HP / 80 Def / 170 SpD
IVs: 0 Atk
Bold Nature
- Scald
- Slack Off
- Psyshock
- Calm Mind
The aim of this set is to stall. Use Calm Mind as soon as possible to improve Slowbro's poor Special Defense. After that, stall with Slack Off and whittle down opponents with Scald and Psyshock. You can consider Iron Defense as an alternative to Psyshock, but you'll appreciate the coverage and the extra damage against Special Defense walls such as Chansey.
Slowbro @ Slowbronite
Ability: Regenerator
EVs: 252 HP / 232 Def / 24 SpD
IVs: 0 Atk
Bold Nature
- Scald
- Slack Off
- Psyshock
- Thunder Wave
This set is meant to be a pure Defense wall. Switch Slowbro in to absorb Physcial attacks and heal off damage with Slack Off. Paralyze foes who depend on their speed; otherwise, use Scald and Psyshock
