Shadow Tornadus (Incarnate Forme)

Shadow Tornadus differentiates itself from its non-shadow forme by.... being only slightly better. Like, to a much smaller degree than what you'd see from most shadows. This is because Shadow Tornadus hasn't been given access to Bleakwind Storm, which basic Tornadus only has access to via special events. Overall, Shadow Tornadus isn't a particularly strong Pokemon and probably isn't worth investing in.

Best PvE Offensive Moveset

• Air Slash and Hurricane is Shadow Tornadus' only real shot at doing much of anything.
• Bite and Dark Pulse CAN work if you're absolutely desperate for Dark coverage. But it's best to not bother.
• Grass Knot and Hyper Beam are basically nothing for Shadow Tornadus.

Air Slash takes advantage of STAB and provides decent energy generation. Bite can be more preferable depending what coverage is needed, but the lack of STAB and very poor energy generation makes it less optimal overall. 

Grass Knot is a strong charge move and covers Rock-type Pokemon that Shadow Tornadus is weak to, along with common Water Pokemon. Dark Pulse provides relatively neutral coverage against various meta picks across all three leagues, but can be particularly useful in formats such as Open Ultra League. Hurricane is quite strong with STAB, but has an expensive energy cost that Shadow Tornadus can't generate comfortably with its fast moves. For that reason, Hyper Beam is also not recommended.