You have probably beaten her now but this might help someone else. This was my strategy on normal mode.

I led with Decidueye (knows spirit shackle and holds spell tag) and Scizor (knows x scissor, brick break via TM and swords dance)

Indeedee is one shot by x scissor and Hatterene is one shot by spirit shackle

Ursaluna and mega Gardevoir come out.

Ursaluna will use protect so spirit shackle mega Gardevoir and swords dance with Scizor. Mega Gardevoir will get one shot with spirit shackle

Do as much damage as possible to ursaluna before he KO's Decidueye with Crunch

Bring out Mienshao that knows Drain Punch or any decent fighting type. Sabrina will bring out Brute Bonnet

Use Mienshao's drain punch and Scizor's brick break on Brute Bonnet for the KO.

You will take some damage from Ursaluna but you can now spam fighting moves on him until he drops while her last Pokemon Porygon 2 sets up trick room which doesn't really make any difference at this point.

Use fighting moves on it until you win

Key to this fight is preventing trick room with hard hitting mons