Question

How do I teach Marshadow the move Ice Punch?

Answer 1

As it is technically part of Marshadow's level up move set, you can teach it with a heart scale by talking to the move reminder at the Mount Lanakila peak Pokémon centre

Answer 2

There are two ways across three Pokemon games in which Marshadow can get Ice Punch.

In SM, Marshadow only gets Ice Punch at level 1.

In PLZA, Marshadow gets Ice Punch at level 1 but also gets it through TM024, which is obtained by achieving Mable's Research level 13.

In USUM, Marshadow still gets Ice Punch at level 1 but can also get it from the move tutor on Ula'ula Beach at Ula'ula Island for 8 BP.

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Source 1: Marshadow's Moveset in Generation 9 - PokémonDB
Source 2: Marshadow's moveset in Generation 7 - PokémonDB

Hope I helped!