Option 1
Slowbro @ Leftovers
Ability: Regenerator
EVs: 252 HP / 252 Def / 4 SpA
Bold Nature
-Slack Off
-Ice Beam
-Scald
-Psychic / Flamethrower / Future Sight
Option 2
Slowbro @ Leftovers
Ability: Regenerator
EVs: 252 HP / 180 Def / 72 SpA
Bold Nature
-Slack Off
-Ice Beam
-Scald
-Psychic / Flamethrower / Future Sight
Slowbro is a solid Physical Wall and can also soft wall Garchomp. The Bro can always switch in a +0 Chomp and check it even if he attacks or set-up.
Option 1 for full utility in a team and Option 2, built in order to check the Shark and OHKO.
Calcs:
Option 1:
+2 252 Atk Garchomp Outrage vs. 252 HP / 252+ Def Slowbro: 264-312 (67 - 79.1%) -- guaranteed 2HKO
252 Atk Garchomp Outrage vs. 252 HP / 252+ Def Slowbro: 133-157 (33.7 - 39.8%) -- guaranteed 3HKO
0 SpA Slowbro Ice Beam vs. 0 HP / 4- SpD Garchomp: 328-388 (91.8 - 108.6%) -- 50% chance to OHKO
Option 2:
+2 252 Atk Garchomp Outrage vs. 252 HP / 180+ Def Slowbro: 280-330 (71 - 83.7%) -- guaranteed 2HKO
252 Atk Garchomp Outrage vs. 252 HP / 180+ Def Slowbro: 141-166 (35.7 - 42.1%) -- guaranteed 3HKO
72 SpA Slowbro Ice Beam vs. 0 HP / 4- SpD Garchomp: 356-420 (99.7 - 117.6%) -- 93.8% chance to OHKO