Streak with Garchomp/Suicune/Ferrothorn is done at 249
For reference, my team and its strategy are here:
Lost on Battle 250 to Clerk Elwin, with a team of:
I was too greedy, trying to switch around on Hydregion and try to get it to lower its special attack with Draco Meteors. An untimely burn on Garchomp from switching back into a Flamethrower aimed at Ferrothorn, coupled with multiple crits on Suicune led to my demise. Yes, there was bad luck, but better play would have avoided the problem.
I fully believe this team is capable of reaching 300, and plan to start a new streak with it, after perhaps dropping Suicune's HP EVs ever so slightly so I can increase speed by a point and avoid the irksome speed ties at 105. I can afford to drop HP by a point without weakening my subs, so the cost seems minimal.