I'd start by giving every levitating pokémon a HA
Gastly line: Shadow Tag
Misdreavus line: Cursed Body
Unown: Magic Guard
Vibrava: Soundproof, Flygon: Sand Stream
Lunatone & Solrock: Sold Rock
Baltoy line: Magic Bounce
Chingling line: Serene Grace
Latios & Latias: Telepathy, M-Latios & M-Latias: Aerilate
Carnivine: Strong Jaw
Rotom: Adaptability (Rotom Fan's regular ability is now Motor Drive)
Uxie: Comatose
Mesprit: Prankster
Azelf: Magic Bounce
Tynamo Line: Galvanize
Cryogonal: Refrigerate
Hydreigon: Sheer Force (I'd give it Berserk if it wasn't Drampa's signature ability, just saying)
Vikavolt: Speed Boost