i'm surprised nobody's contemplated the use of Curse: his Def is great; his Atk is decent; his Spe is low; his SpA and SpD are good enough to stay as is; and his HP is good enough to work with. I'm thinking (this is just being pulled out of my @$$ as i go along)
CurseBro @ Leftovers? Life Orb
Ability: Regeneration, Nature: Brave (+Atk, -Spe)
252 HP/ 252 Atk/ 4 Def
~Curse
~Aqua Tail/Scald/Surf/Brine(DW)
~Earthquake/Zen Headbutt/Brine
~Slack Off/Flamethrower
Max HP EVs to increase bulk. Max Atk EVs and Brave to increase Atk and reduce Spe. Curse is the base of this set. If Life Orb is chosen, you can use Slack Off to heal off any damage taken while beefing up. If Lefties are chosen, Slack Off's kind of redundant, especially with Regenerator, so Flamethrower can take the place of Slack Off for coverage against Bugs and Grass-types. EQ for coverage against Electric types and able to put more of a hurt on Fire-types than Surf or Scald-depending on how many times you've boosted-Zen Headbutt for STAB. Scald(Boiling Water) can be taken over Aqua Tail if you prefer that added Burn chance and accuracy over extra power. Surf for extra damage over Scald, and Aqua Tail to get the boost from Curse. Brine can be used in tandem with Slack Off and whichever item to gain the doubled power under 50% health along with STAB. Curse can be useful if you go physical or even mixed, since his Special stats are useable. if opting out of Slack Off or Lefties, Regenerator can always be put to good use after the boosted Brine.
I'm fairly new to competitive battling, and this set was more or less off the top of my head (i have Slowbro's Bulbapedia page up in another tab), and coming up with sets was never my forte :/ any criticism or improvement is welcome and appreciated