Gotta try a little harder for Battle Tree cheesing XD. Pretty sure that even without negative priority, your strategy could still get destroyed by actual priority though. I don't know the full list of possible opposing 'mons but it would be hilarious if you got hit by Prankster Leech seed.
There's also stuff like prankster taunt shutting him down immediately. Or heck, regular taunt while he sets up rocks.
Then there's also stuff like pokemon with soundproof. If a Kommo-o appeared as an opponent, that would be not only immediate shutdown of roar, but it would also wreck Lucario with stab fighting attacks.
There WAS back in gen… 5? 4?... an infamous Riolu set with roar and copycat that pissed a lot of people off. Basically, you pull off one roar, and then use copycat, which gets priority from prankster (riolu's HA), which then uses the last move used (Roar). Cue annoying, impossible to stop shuffling.
I don't think this works anymore though.