Slowking can't answer Charizard-Y with that spread, since Scald in the sun does squat, and Power Gem can't OHKO. At least the current spread can take a sun-boosted Fire Blast + Solarbeam + Stealth Rock, and do this:
252+ SpA Slowking Power Gem vs. 0 HP / 0 SpD Mega Charizard Y: 284-336 (95.6 -...