I'm assuming that this is for competitive play. Using the Smogon Usage Stats, and that this is RU (Cloyster's tier in Gen 7), Most ice types are already good since they check Donphan, Nidoqueen, Flygon, and Dhelmise, which are the top 4 used Pokemon in UU. But a better Pokemon can resist these Pokemon. The Smogon Donphan set includes EQ, Knock off, and Rapid Spin. Froslass can check this, due to her blocking Rapid Spin, and can use strong, Special moves (e.g. Blizzard) to finish off Donphan. The Smogon set for Nidqueen consists of Earth Power, Ice Beam, and Thunderbolt/Sludge Wave. Assuming that most people will use Sludge Wave over TB due to STAB, a Pokemon that can check this is Jynx. Notice that I said CAN. Jynx has a higher SpA than Nidoqueen's SpD, is faster, and both STAB moves are Super Effective against Nidoqueen. Next is Flygon. Flygon has a 4x weakness to Ice and has equal Def/SpD stats. The Smogon set has Iron Tail, Outrage, and EQ. Eviolite Piloswine can check this (notice the word can again.) If fully invested in Def and Atk, it may give Flygon a hard time. It has access to Ice Shard (Priority) and other Ice moves. Also, only Iron Tail is Super Effective against it, and it only hits 3 out of every 4 times. Finally, Dhelmise's Smogon set is okay, but I would like to base it off Dhelmise in general. Dhelmise get Steelworker, so there is a higher chance of a Dhelmise you meet use a Steel-type move. If you assume it will have one of each of it's STAB and Anchor Shot and Rapid Spin, Cloyster could work. Skill Link is a useful ability, and with moves like Icicle Spear, it could work. Cloyster also gets access to Twin Needles, which can be very useful. Only Grass is Super Effective, but Cloyster is faster, and if it is Admant and has 252 EVS in Attack, Icicle Spear/Skill Link will OHKO a Dhelmise fully invested in Defense with a helping nature.
Sorry if it is a bit long/confusing! I worked really hard and long on this answer, so I hope it helps!
Source: Damage Calc
Source: Smogon
Source: PS!
Source: The DB
