Check:
Entei @ Heavy-Duty Boots
Ability: Pressure
Tera Type: Dark
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Sacred Fire
- Extreme Speed
- Stone Edge
- Crunch
Entei is a check to armarouge. With stone edge or crunch, which helps deal with other good mons such as slowbro and moltres, entei can deal serious damage to armarouge, while still being able to take even a +1 psyshock easily. If armarouge decides to tera fairy, stone edge is still doing 50% to it, and the defense drop means that extreme speed is going to be picking it off with a bit of chip beforehand. Tera dark means that entei can avoid psyshock if needed.
Counter:
Arcanine @ Heavy-Duty Boots
Ability: Flash Fire
Tera Type: Dark
EVs: 252 HP / 252 Atk / 4 SpD
Adamant Nature
- Morning Sun
- Extreme Speed
- Flare Blitz
- Crunch
Arcanine is a counter to armarouge. It of course has flash fire in order to deal with armour cannon and with max hp investment, can never be 2hit ko'd by psyshock from full. From here, arcanine can recover off the damage with morning sun and then hit armarouge with crunch, which does big damage and puts it into e-speed range. Conversely, if they tera fairy on the crunch turn, then they have a good chance to be ohko'd by flare blitz afterwards. Tera dark means in a pinch you can become immune to psyshock.