Mega Charizard Y
Mega Charizard Y brings its own sun with Drought, which powers its Fire STAB by 50% and turns Solar Beam into a one-turn nuke. Off its huge Special Attack, a sun-boosted Heat Wave or Overheat threatens to OHKO or 2HKO most neutral targets before you can set up, and Solar Beam punishes the Water/Rock that would normally wall it.
Don't leave your Steel or Grass answer in front of it in the sun — a sun-boosted Fire move that would be a safe switch under neutral weather can outright OHKO. And don't bring a rain/sand plan that loses the weather war to Drought; if the sun stays up, so does the nuke.
Rock resists Fire and Flying and Rhyperior's bulk eats a sun-boosted Heat Wave, threatening a Rock STAB OHKO back. Watch for Solar Beam — but forcing Charizard onto its Grass move is already a won turn, since it's not clicking Fire that turn.
Even in sun, Archaludon's special bulk (especially with Assault Vest) lets it survive Heat Wave and answer with Flash Cannon/Draco. It doesn't fold to one nuke the way frailer Steels do.
Milotic resists Fire and can bring rain-independent bulk; if your team can swing the weather back (or just tank the sun hit), the sun-boost evaporates and Charizard becomes a manageable special attacker. Haze also erases any Overheat drops working in its favor.
Charizard-Y wants a partner that lures or removes its checks (Solar Beam handles Water/Rock, a teammate handles Steel). Keep your weather-setter or bulky Water alive so the sun and the nuke don't get a free turn to snowball.
Every answer on this card was checked against the live learnset and roster data shown above — a citation that no longer holds is dropped, not shown. Type-based answers are computed from base stats and STAB. Speed is the real level-50 maximum. This is deterministic and sourced — no guesses.