
:
Cleanly 2hkos with Surf:
228+ SpA Tentacruel Surf vs. 144 HP / 196+ SpD Arcanine: 212-252 (59.3 - 70.5%) -- guaranteed 2HKO
Tanks an Outrage, and is not 2hko'd by Espeed:
168 Atk Choice Band Arcanine Outrage vs. 244 HP / 8 Def Tentacruel: 229-270 (63.2 - 74.5%) -- guaranteed 2HKO
168 Atk Choice Band Arcanine Extreme Speed vs. 244 HP / 8 Def Tentacruel: 153-180 (42.2 - 49.7%) -- guaranteed 3HKO

:
Acid spray into Hyper Beam:
228+ SpA Tentacruel Acid Spray vs. 248 HP / 36 SpD Kyurem: 54-64 (11.9 - 14.1%) -- possible 8HKO
228+ SpA Normal Gem Tentacruel Hyper Beam vs. -2 248 HP / 36 SpD Kyurem: 400-471 (88.3 - 103.9%) -- 25% chance to OHKO
11.9+88.3=100.2% minimum
Draco doesn't ohko:
40+ SpA Choice Specs Kyurem Draco Meteor vs. 244 HP / 0 SpD Tentacruel: 274-324 (75.6 - 89.5%) -- guaranteed 2HKO

:
Interesting interaction between Calm Mind and Mirror Coat, but you essentially just need to Acid Spray until Suicune gets to at least -3:
228+ SpA Normal Gem Tentacruel Hyper Beam vs. -3 252 HP / 0 SpD Suicune: 422-497 (104.4 - 123%) -- guaranteed OHKO
You absolutely tank whatever Scald's come your way:
0 SpA Suicune Scald vs. 244 HP / 0 SpD Tentacruel: 33-40 (9.1 - 11%) -- possibly the worst move ever
Assuming maximum damage for every acid spray, Mirror Coating every turn won't save Suicune either:
228+ SpA Tentacruel Acid Spray vs. 252 HP / 0 SpD Suicune: 46-55 (11.3 - 13.6%) -- possible 8HKO
Possible damage amounts: (46, 46, 48, 48, 48, 49, 49, 51, 51, 51, 52, 52, 52, 54, 54, 55)
(Mirror coat deals 110 maxium)
228+ SpA Tentacruel Acid Spray vs. -2 252 HP / 0 SpD Suicune: 91-108 (22.5 - 26.7%) -- 23.2% chance to 4HKO
Possible damage amounts: (91, 91, 93, 94, 96, 96, 97, 99, 99, 100, 102, 103, 103, 105, 106, 108)
(Mirror coat deals 216 maximum)
With 362 HP, Tenta always lives max roll Mirror coats (110+216 = 326)