Tornadus-Therian is underwhelming. Base 238 Attack isn't terrible, but a lackluster Flying-type moveset in Air Slash/Hurricane means that Tornadus-T performs worse than Moltres, Honchkrow, Rayquaza, and even Unfezant (the Pidgey of Gen 5), while requiring Rare Candies. Bite/Dark Pulse isn't a bad Dark-type moveset, but Tornadus-T lacks STAB, meaning it performs worse than Houndoom and Absol, let alone the stronger Dark-types like Weavile and Hydreigon.
Level 40 Rampardos is the only Pokemon that can duo Tornadus. If you do not have 6 maxed out Rampardos, this guide will also cover the trio, which is much easier.
Minimum viable levels assume Best Friends, no Weather, no Dodging.
Italics indicate a Legacy move.
Level 40 Rampardos is the only viable option for duoing Tornadus without Weather.
Dodging Charge Moves will make the duo much easier, as you can cut down the total death count to 10-11, or 5-5.5 deaths per trainer, allowing you to avoid relobbying entirely.
Minimum viable levels assume Best Friends, no Weather, no Dodging.
Italics indicate a Legacy move.
The Tornadus trio is much, much easier than the duo. Almost any Ice-, Electric-, or Rock-type found on this list, powered up to Level 25-35, will be able to trio.
Electric-types perform very well against Hurricane compared to other movesets, thanks to their resistance to Flying.
Yes, Aggron can trio Tornadus-T. No, this does not mean Aggron is a good attacker. The fact that Aggron can trio Tornadus-T is a testament to how easy the Tornadus trio is, rather than how good Aggron is.
With no Dodging, Rampardos is non-viable for the trio against Grass Knot under Level 35, due to how many times it will faint at low levels.
If you can dodge all Grass Knots, however, Rampardos is trio-viable at Level 20.