I know I have wing attack and he works great but I only have one so hopefully someone else can vaulge for air slash.
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Second this. Mine has WA and it charges FT quickly. Also works well if you're a dodger like myself.
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I have both a WA/FT and a AS/OH and I prefer the WA variant on offense
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Air Slash is stronger than Wing Attack on defense. They have similar damage on offense, but Air Slash has a longer duration, so it is harder to dodge with.
The bigger difference for Charizard is with the charge move. On offense, Flamethrower and Fire Blast are close, and Overheat is a lot stronger.
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I have one with WA/FB and the second with AS/OH. I am trying to decide if I should keep both as the one with WA has decent IV (78%) while the one with AS has great IV (96%).
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If you like to dodge, Wing Attack and Ember are both shorter.
If you are a screen masher, Air Slash and Fire Spin give you access to Overheat.