0 Atk Feraligatr Ice Fang vs. 0 HP / 0 Def Meganium: 100-118 (37 - 43.7%) -- guaranteed 3HKO
0 SpA Feraligatr Ice Beam vs. 0 HP / 0 SpD Meganium: 110-130 (40.7 - 48.1%) -- guaranteed 3HKO
As you can see, Ice Beam will do more damage so use ice beam
Edit: I'm actually really surprised ice beam does more. I thought Ice fang would be stronger.
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