Seeing Tentacruel in the Checks & Counters sounded kinda off to me, so I ran some damage calcs and came up with this.
+2 252+ Atk Cloyster Rock Blast (5 hits) vs. 252 HP / 240+ Def Tentacruel: 230-275 (63.1 - 75.5%) -- guaranteed 2HKO after Black Sludge recovery
If Cloyster already has a Shell Smash up, Tentacruel isn't really that great of a counter considering that Rock Blast chops off almost 2/3s of its health from one turn, while Tentacruel can only retaliate with this:
0 SpA Tentacruel Scald vs. -1 4 HP / 0 SpD Cloyster: 100-118 (41.3 - 48.7%) -- guaranteed 3HKO
Even if Cloyster gets burned, it's still going to KO Tentacruel unless it protects after the initial Rock Blast, and this is assuming that Tenta's at full health without any entry hazards.
Plus, Tentacruel couldn't switch in and then tank a hit unless Cloyster's using Hydro Pump or setting up the initial shell smash, so it requires a teammate to faint or maybe a slow -turn to get into the battle in the first place.
Also, if Cloyster's holding a Lum Berry, Tentacruel's Toxic or Scald burn won't even slow it down, and that 41% flinch chance with can be a huge liability considering Tentacruel's low physical bulk. On the off chance that Cloyster has a White Herb..
0 SpA Tentacruel Scald vs. 4 HP / 0 SpD Cloyster: 67-79 (27.6 - 32.6%) -- guaranteed 4HKO
Heck, I really like Tentacruel, but it's much more of a shaky situational check than a solid counter.