Flame-flare-axe only does just under 300 damage. If the Heavy is at all overhealed, you will not kill him. If he has a Medic on him and is only partly overhealed, flame-axe-axe will kill him, but not flame-flare-axe. Much more reliable in that sense imo.
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_foolmo_
'I mean, what does electricity have to do with math besides calculators?' - ZeldaFreak29