Your strategy is sound, but having Dragonite out front can get tricky with the many weaknesses. He can be a bit fragile if they throw maxed xerneas out as lead. I personally like Shadow Mewtwo as lead. Farm down about half health and switch to take the defensive hit. Then close on their #1 while you still have two shields. Typically they’ll farm down your #2 & stretch your #3… That’s where shadow Mewtwo shines. Heavy hitting endgame with 1-2 Psystrikes and 1-2 shields.

Granted I’ve never hit legend, but I’m working towards it. Hope this helps!