Emboar should not use Impish nature; it is not so hot for stalling, so why not using its amazing attack to make it hits hard like a truck ? Here is a better moveset:
@ Expert Belt:
EV: 252 Atk / 252 HP / 4 Def
- Flare Blitz
- Wild Charge
- Hammer Arm
The EV spread gives Emboar the role of a physical tank. Adamant Nature gives it extra attack, although if you really cannot change your natures, then keep Impish, although not recommended. Flare Blitz is a truck-like hitting STAB, abusing Emboar's power to make some massive damage. Earthquake has a wide variety of attacking super effectively, so that the Expert Belt is abused. Wild Charge is some good check against both Water and Flying Pokémon. Finally, Hammer Arm is the second STAB, also hitting very hard.
This one, except if you use Thunder and Surf, should never, never and never be of any nature that raises the Special Attack stat.
@ Normal Gem:
EV: 252 Atk / 252 Speed / 4 HP
- Wild Charge
- Fire Fang
The EV spread make Stoutland a physical sweeper and the Jolly nature augments its decent speed. Return needs you to make Stoutland love you, and is impressive with the Normal Gem but I would prefer a Choice Scarf over the Normal Gem if possible. Wild Charge basically do the same as Emboar's, covering Water and Flying. Fire Fang attacks Steel Pokémon as well as Grass, Bug and Ice, which is good, and Crunch hits ghosts.
It does not learn Drill Peck. I would chose Jolly > Quirky for the extra speed, and would have a moveset like this one:
Trait: Big Pecks
EV: 252 HP / 252 Speed / 4 Atk
The main reason I prefer this moveset to a super offensive one is to give you a supporter. The EV spread is supporting what I want to do with Unfezant, and the Jolly nature gives extra speed, which is pretty much needed.
Fly is the main attack. Generally, it sucks, but here, it is needed for one reason: Toxic. Toxic deals continuous damage and while in air, the foe cannot attack you normally. FeatherDance and Swagger combo together pretty well to reduce any Physical based Pokémon to oblivion.
A modest Zebstrika is pretty good. I'll just change its moveset into this:
@ Electric Gem:
EV: 252 Sp.Atk / 252 Speed / 4 HP
- Volt Switch
It has very few options. The EV spread transforms the Zebra into a Special attacking Pokémon. Overheat deals massive damage then weakens you. You must switch after it. Volt Switch is what will let you both switch and deal damage. Thunderbolt is the main STAB. Pursuit is a filler.
Its nature is OK, I can render it a Mixed Attacking Pokémon.
@ Big Root:
Trait: Poison Touch
EV: 252 Atk / 252 Sp.Atk / 4 HP
- Drain Punch
- Muddy Water
- Grass Knot
The EV spread and the nature makes together a good use of Seismitoad. Earthquake is the main STAB, Drain Punch is the key: It activates Big Root, for healing, deals decent damage, and with Poison Touch, it has some chances on poisoning. Muddy Water has a great BP, but a lackluster accuracy. Finally, Grass Knot is a choice, among others, for hard-hitting obese Pokémon.
I'll make it mixed, like Seismitoad.
@ Flying Gem / Salac Berry:
EV: 252 Speed / 128 Sp.Atk / 128 Atk
Naive / Lonely Nature
- Energy Ball
- Rock Slide / Focus Blast
You can either chose between the Flying Gem, to strengthen *Acrobatics, or Salac Berry, part of the SubSalac Simisage. But if you don't have access to Salac Berry, then Flying Gem, and remove Substitute for getting both Rock Slide and Focus Blast, 2 respectable coverage move. Energy Ball is the STAB. The EV spread makes Simisage a good and quick mixed attacker, and the nature's choice is up to you: Lonely for extra power, Naive for extra speed.
Hope I helped!