I think normal convention is to just power great attackers up to level 30. So level 36 is probably overkill, but if you've got a ton of stardust...
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Just stay at level 30 as an attacker bro...
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I feel like it depends on the attacker. I'm currently level 32 and I have fully powered up my Jolteon, Victreebell and, Golem though I am probably done leveling the Golem. The other two need as much health as they can get.
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Against Charizard, you don't really need a level 36 Pokémon, save your candies and stardust for Machamp and others.
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Correct for dust. There are only rare other applications for Golem-Candy. Machamp does not like those unfortunately.