Well, I feel it's pretty obvious we could use something to stop us from being Fighting and Ground weak, particularly since EQ spam can be quite effective. Luckily, Doubles is full of mons that can deal with these things. We also want something that doesn't really mind Will-o-Wisp, since Bisharp and Terrakion both do. Three super common Pokemon that come to mind are Cresselia, Charizard, and Togekiss.
I'd like to see us go a slightly less standard route.
Salamence @ Choice Scarf
Ability: Intimidate
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
- Draco Meteor
- Dragon Pulse
- Hydro Pump
- Fire Blast
Finally! An Intimidate user that doesn't hate every other Intimidate user or Will-o-Wisp. Mence has a good Speed tier for Doubles, a great ability, and a surprisingly high Special Attack of 110, for those who don't remember the terror that was MixMence. Since neither Bisharp nor Terrakion mind Rock Slides, there are already 2 good switch-ins for that. Additionally, Bisharp and Terrakion do a good job of beating down Hail. Meanwhile, Salamence provides a switch in to Fighting and Ground moves, as well as a nice Fire and Water resist.
tl;dr good type synergy, fast, intimidate, special attacker