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
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