Tornadus Pokémon GO counters are important to know if you want to take down the Tier 5 raid boss. Tornadus is powerful, but straight forward to counter.
Tornadus holds the distinction of being the only pure Flying type in Pokémon Go. As such, it is weak to Ice, Electric and Rock type moves.
Tornadus Pokémon Go Counters: How to Beat the Cyclone Pokémon
Mamoswine with Ice moves is a good counter against Tornadus. It will be resistant to any Ground moves your Mamoswine might know, so be sure to use an Ice focused build against it. Weavile and Glaceon are also acceptable Ice wielders to use against Tornadus.
Electivire and Raikou are good Electric-type Pokémon you should already have around if you're a serious Raider. These are the best Electric types you could get your hands on, and cover a lot of weaknesses Legendary Pokémon like Torndaus have.
A good Rock type attacker will also get you through. Rampardos is one of the best Rock type Pokémon, and its pre-evolution Cranidos is currently being featured in the Fossil Event. This would be a good time to get a strong Rock type if you don't already have something like Tyranitar.
Keep in mind that Tornadus can learn Grass Knot, which would put your Rock type Pokémon in danger. When your game recommends a team, if it doesn't give you a Rock type it's likely the Tornadus you're fighting will have Grass Knot. You should bring Electric or Ice type Pokémon with you instead.
Tornadus shouldn't give you any trouble as long as you're using Pokémon that can deal super effective damage against it. Most of the Pokémon that can deal the damage to Tornadus also luckily resist its Flying type moves.