Ability: rain dish
EVs: 252 HP, 151 sp. attack, the rest in speed or sp. defense
Barrier: used to make it completely tanky.
Rain dance:when coupled with black sludge, it restores 1/8 of tentacruel's max HP for 5 turns due to rain dish. An alternative if rain is already set up is mirror coat or infestation for hitting special attackers and getting chip damage, respectively.
Sludge bomb: STAB, good chance to poison, good power. An alternative is mirror coat.
Giga drain: even more HP recovery and counters water types while having good power. An alternative is scald for water fire coverage instead of water coverage
[email protected] orb
252 Evs in speed and attack
Scald: chance to burn, good power, STAB
Sludge bomb: same reasons as above
Knock off: kills psychics, knocks of items
The last slot is up to u