They can still switch in and threaten with a huge amount of damage if not a downright KO, though.
252 SpA Keldeo Surf vs. 4 HP / 0 SpD Entei: 372-440 (100 - 118.2%) -- guaranteed OHKO
That's without Expert Belt, LO, Specs or anything. It faints. 100% of the time. It outspeeds you on the first turn, even if you get the burn on it, which is still only 50%, assuming it hits [which it probably will]
252 Atk Terrakion Stone Edge vs. 4 HP / 0 Def Entei: 372-440 (100 - 118.2%) -- guaranteed OHKO
Same thing, straight-up dies, though shouldn't try to switch in, just in case.
Char gets Roost, yo.
252 Atk Tough Claws Mega Charizard X Dragon Claw vs. 4 HP / 0 Def Entei: 195-231 (52.4 - 62%) -- guaranteed 2HKO
Who's the lucky lady that outspeeds?
252+ SpA Mega Launcher Mega Blastoise Water Pulse vs. 4 HP / 0 SpD Entei: 422-498 (113.4 - 133.8%) -- guaranteed OHKO
Admittedly, though, Entei gets ~40% on it if they're facing each other one-on-one and it ends up getting the burn.
4 SpA Slowbro Scald vs. 4 HP / 0 SpD Entei: 218-260 (58.6 - 69.8%) -- guaranteed 2HKO
Meanwhile, Your best attack does a third, and it's Stone Edge. The bro prefers the burn to toxic poison, and can switch out to take advantage of Regenerator.
I'd answer the Sacred Fire/Flare Blitz thing, but you guys seem to have covered that.