That is not very overpowered. Sturdy users would become immune to Raticate's fang moves, and Tyranitar is immune to any attack. With Victini, you can't lose a battle. The former two are solid counters, and the latter 2 are way more OP. Skarmory can roost off all damage and 2HKO with Brave Bird, while Sturdy makes it immune to OHKO moves. So sturdy Skarmory hard counters Raticate. Tyranitar can't really be harmed, and OHKOs with Stone Edge.Raticate's fang moves (Fire Fang, Super Fang, Hyper Fang, etc.) always OHKO.
Besides, it can't do anything to Sturdy Skarmory.
252+ Atk Raticate Wild Charge vs. 252 HP / 0 Def Skarmory: 118-140 (35.3 - 41.9%) -- 86.5% chance to 3HKO after Leftovers recovery.
252+ Atk Raticate Flame Wheel vs. 252 HP / 0 Def Skarmory: 78-94 (23.3 - 28.1%) -- possible 5HKO after Leftovers recovery
0- Atk Skarmory Brave Bird vs. 0 HP / 0 Def Raticate: 145-172 (57.7 - 68.5%) -- guaranteed 2HKO
252+ Atk Tyranitar Stone Edge vs. 0 HP / 0 Def Raticate: 279-328 (111.1 - 130.6%) -- guaranteed OHKO