Shadow Cloyster

Shadow Cloyster is in a bit of a bad position. It loses a huge chunk of its defense when compared to its normal forme, which when combined with its fairly low HP hurts it a bit. It's an okay attacker, but overall it's best to ignore it and raise a Mamoswine instead.

Best PvE Offensive Moveset

• Frost Breath and Ice Shard perform similarly, but Frost Breath has some slight advantages.

• Avalanche is a potent 2-bar charge move that outperforms Blizzard, Shadow Cloyster's next best charge move.

• Hydro Pump and Liquidation, while being good moves with STAB, don't share the same typing as Cloyster's fast moves.

• Aurora Beam and Icy Wind deal mediocre damage with long cooldown. There's no reason to use these moves.

Best PvE Defensive Moveset

• Between Ice Shard and Frost Breath, Ice Shard does more damage on defense. 

• Liquidation is the obvious superior charge move due to having 2 bars, STAB, and high power. 

• Razor Shell has less power than Liquidation, but is harder to dodge.

• Avalanche is okay, but sharing the same type as Shadow Cloyster's Fast Moves is a bit of a hinderance.

• Hydro Pump is off-type with Shadow Cloyster's fast moves and hard to dodge, but Shadow Cloyster may not get it off.

• Aurora Beam and Icy Wind are functional for having multiple bars, but aren't nearly as good as Avalanche.

• Compared to Avalanche and Hydro Pump, Blizzard is worse on defense in nearly all respects.

Ice Shard is chosen over Frost Breath due to its superior energy generation. Icy Wind is a move as powerful as it is annoying, hitting fairly hard while also debuffing its target's Attack stat. As for your secondary move of choice, it mostly comes down to preference. Avalanche locks you into a mono-Ice moveset, which can be problematic. Hydro Pump is expensive, and often hard to reach, but does provide additional coverage.