Despite what the ubiquity of its pre-evolution in the wild may suggest, Tentacruel is a highly valued Pokemon in the OU metagame, being a stellar defensive Rapid Spin user. Unlike other defensive spinners, Tentacruel is particularly resistant to being worn down, as it can take advantage of rain support to get 12.5% healing per turn with Rain Dish and Leftovers combined. One of the major selling points of Tentacruel is that it can beat any spinblocker under rain with a combination of Substitute and Toxic; this includes Sableye, and due to this, Tentacruel is an extremely good spinner to use on rain teams. Alternatively, Tentacruel can use Toxic Spikes, or it can tank hits with its good Special Defense and semi-unique Water/Poison typing. However, its gelatinous exterior is vulnerable to physical assaults due to an unfortunately average Defense stat for a support Pokemon. It is also encumbered by a few weaknesses to common attacks such as Earthquake, Thunderbolt, and Psyshock, which somewhat limit its defensive potential. Nonetheless, Tentacruel remains the Rapid Spin user of choice for many teams, whether they are utilizing rain or not.
This set excels at spinning away hazards for both balanced and stall teams alike. Unlike other users of Rapid Spin, Tentacruel is able to absorb Toxic Spikes, facilitating its work as a spinner. Tentacruel isn't limited to spinning either—it can spread status and stall out many threats.
Although Tentacruel has access to Toxic Spikes, Toxic is still the preferable move. This is because many teams benefit more from the immediate poison, and there are also many Flying-types and Levitate Pokemon that can only be poisoned by Toxic. Toxic also has other advantages, such as allowing Tentacruel to consistently defeat most relevant spinblockers in rain. Nevertheless, Toxic Spikes is still a viable option, and Tentacruel has a niche as one of the few viable users of this move. Toxic Spikes is preferred on stall teams, or when using a sweeper that benefits from certain walls being crippled, particularly those with Natural Cure.
The third slot goes to Scald, the best STAB move that Tentacruel can run. It has the same power under rain as a Hydro Pump outside of rain and has a 30% chance to burn the target, discouraging Pokemon such as Gyarados and Dragonite from switching in, even though they resist the move. The move in the fourth slot depends on your preference. Protect is the main choice, as it allows Tentacruel to stall for more Leftovers + Rain Dish recovery. Protect also has the ability to scout Choice users and stop SubDisable Gengar from ruining your day. Another good option is Substitute, which blocks status and Leech Seed, and most stall teams typically have trouble breaking Tentacruel's Substitutes. When using Substitute, remember to only use it with Toxic, and not Toxic Spikes.Team Options & Additional Comments >>>
Tentacruel can run an offensive Rapid Spin set holding a Life Orb with a Timid nature, an EV spread of 252 SpA / 4 SpD / 252 Spe, and a moveset of Hydro Pump, Rapid Spin, Ice Beam, and Giga Drain. The main reason why this set is not listed above is because it is mostly outclassed by Starmie, which has higher Speed and Special Attack, access to reliable recovery, and a superior offensive movepool. The surprise value and unique typing gives it a small degree of viability, though.
Liquid Ooze may come in handy when confronting Pokemon such as Conkeldurr and Ferrothorn, which can use Drain Punch and Leech Seed, respectively. It also seems like an appealing option to use outside rain teams, since there is no rain to activate Rain Dish. The latter is still, however, the superior option. Even if you are not using a rain team, you are able to take advantage of your opponent's rain.
Knock Off is an interesting utility move, especially considering that it can make Leftovers users succumb quickly to poison. However, Tentacruel has trouble finding room to accommodate this move. Confuse Ray deals even more damage to the opponent if you are lucky, but Tentacruel has more consistent options to run. Magic Coat can block Taunt and status moves, but on Tentacruel, this move is rather gimmicky. Acid Spray could be used to force out opponents in fear of taking attacks at -2 Special Defense, but Tentacruel still isn't very threatening offensively.
Tentacruel can actually go to the sweeping route by using a SubSD set with Substitute, Swords Dance, Waterfall, and Poison Jab. Despite a paltry base 70 Attack stat, Tentacruel will still hit hard after a Swords Dance or two, and Water- and Poison-type attacks have surprisingly good coverage together. That said, despite Tentacruel's good speed and longevity under rain, it tends to be outclassed by other physical sweepers due to its shallow physical movepool and the fact that this set is often inferior to Tentacruel's other sets.
Checks and Counters
If Tentacruel is running Toxic Spikes, anything that is immune to them and doesn't take much damage from Scald can be considered a counter. Reuniclus, Latias, Latios, and Hydreigon are prime examples. The former two even have super effective STAB moves to use against Tentacruel. Dragonite and Gyarados actually fit the above description, but they will hate being burned, unless they run Substitute or Lum Berry.
Poison Heal Breloom is one of the best counters to Tentacruel, as it benefits from being poisoned and can use Tentacruel as setup fodder. However, be wary that Breloom is vulnerable to Scald burns if it doesn't have its Toxic Orb activated and Toxic Spikes aren't on the field. Celebi is probably the best counter to Tentacruel, as it can simply cure itself of status when switching out, takes little damage from most of Tentacruel's attacks, and can easily dispose of it with Psychic. Amoonguss absorbs Toxic Spikes and takes almost no damage from Scald, but it cannot retaliate aside from using Spore. Venusaur can also remove Toxic Spikes, set up Growth, and mitigate potential damage from Ice Beam or Scald burns with Giga Drain. Toxicroak is completely immune to everything in Tentacruel's arsenal and can remove Toxic Spikes. Even though Tentacruel resists both of Toxicroak's STABs, it may be overpowered after a few boosts.
SubCM Latias and SubCM Keldeo aren't bothered by anything that Tentacruel can throw at them and can set up on it easily if they haven't been poisoned by Toxic Spikes or by Toxic on the switch-in. Both Alakazam and Reuniclus do not have to worry about status at all, and can easily dispose of Tentacruel with their STAB moves, although Alakazam takes decent damage from Scald due to its natural frailty. Starmie can spin away Toxic Spikes and can force Tentacruel out with the threat of STAB Psyshock, Thunderbolt, or Thunderbolt. Natural Cure means that Starmie does not have to worry about status. If Tentacruel lacks Toxic or has not set up Toxic Spikes, Jellicent has no problem coming in to block Rapid Spin and can beat it one-on-one with Taunt, Will-O-Wisp, and Recover.
If the weather is sunny, Heatran takes little damage from Scald and can defeat Tentacruel with a super effective Earth Power. Speaking of Earth Power, any Pokemon with that move or Earthquake will easily force Tentacruel out. Examples of this are Ground-types such as Landorus, Landorus-T, Mamoswine, and Garchomp. However, they all loathe burns, and the first two are weak to Scald. Gastrodon can switch in on any of Tentacruel's moves (barring Toxic) and demolish it with Earthquake or Earth Power. Electric-types, like Ground-types, also have super effective STABs and can easily force Tentacruel out. Zapdos and Magnezone can withstand a Scald and retaliate with powerful Thunderbolts/Thunders. Thundurus-T will easily force Tentacruel out, which may give it the perfect opportunity to set up a Nasty Plot or Agility. However, Thundurus-T has difficulty switching in on Scald or Toxic; the former does heavy damage, and the latter shortens its lifespan even further. SubDisable Gengar is immune to everything Tentacruel has bar Scald and can spinblock. Once it has a Substitute under its belt, it can disable Scald and Shadow Ball Tentacruel to death. A smart player can get around this by using Protect, though. Gothitelle can switch in on Tentacruel with minimal risk, trap it with Shadow Tag, and OHKO with Psyshock or Psychic. Dugtrio can also do the same with Arena Trap and Earthquake, but it needs Tentacruel to be slightly weakened, and even with Focus Sash, it cannot really switch in on Tentacruel's Scald, because if it is burned, Dugtrio loses.