Quality Checks & Counters! I have more to add, though:
~ Gastrodon can switch in on any moves barring Toxic and can demolish Tentacruel with EQ / Earth Power.
~ Toxicroak can switch in on anything and start setting up
~ Add SubCM Keldeo with Latias.
~ Mention that Gengar can use Thunder to quickly dispatch Tentacruel. Add Jellicent & Sableye, too, since they prevent Tentacruel from spinning if it lacks Toxic.
~ Either on your Sun description or Breloom / Celebi, mention how Venusaur can set up Growth on Tentacruel / absorb Toxic Spikes
~ In the same description with Dragonite, include Kyurem / Kyurem-B as other scary threats that can set up on Tentacruel
~ Add Garchomp to your Ground-types; Dugtrio can trap and finish off Tentacruel at ~50% health
~ Add Zapdos and Magnezone alongside Thundurus-T; with Roost and some special-defense investments, Zapdos can shrug off Scald burns if it comes to that. Magnezone can start firing off strong Thunders.
~ Add Gothitelle, a Pokemon that can easily switch in with low risk and consistently eliminate Tentacruel.
~ Remove Sigilyph and SDef Jirachi; Sigilyph is not relevant in OU, and Jirachi does not enjoy getting burned.
Also please rephrase / make changes that I suggested here