I feel Quagsire may be pretty reliable.

Shell Bell / Rindo Berry (Only if he has Trailblaze)
Water Absorb (If he doesn't have Scald then go for Unaware in case he has Bulk Up)
Tera Ground
252 Atk / 252 Def / 6 HP
Adamant Nature
Earthquake
Chilling Water
Curse
Recover
Because of how weak Trailblaze is, combined with the lack of STAB, with the berry it leaves Hammer Arm as Emboar's strongest move pre Tera, and after Tera Flare Blitz.
0+ Atk Tera Electric Emboar Trailblaze vs. 6 HP / 252 Def Rindo Berry Quagsire: 84-100 (25.3 - 30.1%) -- guaranteed 4HKO
0+ Atk Tera Electric Emboar Hammer Arm vs. 6 HP / 252 Def Quagsire: 124-147 (37.3 - 44.2%) -- guaranteed 3HKO
0+ Atk Reckless Tera Electric Emboar Flare Blitz vs. 6 HP / 252 Def Tera Ground Quagsire: 178-211 (53.6 - 63.5%) -- guaranteed 2HKO
I know those Flare Blitz numbers aren't exactly pleasant, but from all the Chilling Waters and Curses you should weaken it enough to where it doesn't become a problem.
Clodsire could also probably work, but I'd rather not try taking a whole barrage of Flare Blitz that could remove half my health.