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I do understand the huge damage difference, and forgot about dry skin. After all of that I mean it has to be close, honestly you could also put like a mega charizard-Y with blast burn, but you would also lose 30 power and no item, or we could even do like mewtwo Y, with reflect type and mimic. But after all that, I think it's probably still shuckle just because of the imense now 230 attack. We could even max attack and reflect a fire type, and mimic the move, and do that, I feel that that would do more damage, but it still could probably be rollout.
If we used shuckle, we would have an immense 2456 attack, with the blast burn, sunny day, STAB with adaptability, or just almost 5000 attack with huge power. dry skin, and 8x super effective. I feel this would do more damage than everything else. Obviously its just numbers, and very complicated, and very particular, and wont ever happen. But it's very interesting to me, just to realize that much damage.