Cloyster

Cloyster is a fairly common Pokemon that can provide solid Ice-type damage. However, it's badly outclassed in this capacity by the other big-name Ice Types. Although Cloyster has an outstanding defense stat, it's backed up by embarrassingly low HP, which makes Cloyster's bulk merely above average. It's not a complete pushover on defense due to its typing, which is difficult to counter outright.

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, Cloyster's next best charge move.

• Hydro Pump, Razor Shell, 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 Cloyster's Fast Moves is a bit of a hinderance.

• Hydro Pump is off-type with Cloyster's fast moves and hard to dodge, but 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 the preferred fast move due to its better energy gains, which is necessary to take advantage of Cloyster's strong charge moves. 

Icy Wind and Avalanche both cost the same 45 energy, which makes it easy to keep the opponent guessing. The former has poor damage output but Attack debuff utility, while the latter threatens much stronger damage. Liquidation offers coverage at a moderate cost. Hydro Pump is generally too expensive.