Sorry to be that guy, but...
Why was this the set instead of the Rocky Helmet one? Your set gets 3HKO'd by Stone Edge (69% chance by Crunch after Sandstorm damage) and can't actually OHKO in Sand because Rock-types get the SpDef boost. Here is calc:
0 SpA Tangrowth Leaf Storm vs. 0 HP / 0 SpD Tyranitar: 204-240 (59.8 - 70.3%) -- guaranteed 2HKO (so this factors in Sand because TTar is assumed to have Sand)
Essentially what would happen is, you come in on a Stone Edge/Crunch, take a Stone Edge/Crunch as you Leaf Storm, and then die to Stone Edge (possibly living Crunch) and then you KO with EQ (assuming you live). I guess you could switch out before after Leaf Storm, but I wouldn't really call it a counter.
EDIT: Oops I'm supposed to post an appropriate counter.
Landorus-Therian @ Choice Scarf
Ability: Intimidate
EVs: 252 Atk / 4 Def / 252 Spe
Jolly Nature
- Earthquake
- U-turn
- Stone Edge
- Knock Off
TTar Stone Edge 3HKOs at best after Intimidate, while Lando definitely 2HKOs with EQ (and is faster).