Ghost Types: Ghost Types are immune to Endeavor thus nullifying the strategy.
Entry Hazards: They render Sturdy and Focus-sash useless and thus making them capable of being OHKOed
Status conditions: Particularly poison, and burn they make them take damage without lowering them down to 1.
Mold-breaker: Nullifies Sturdy so you can OHKO
Multi-hit moves: They hit multiple times so Sturdy and Focus-sash make no difference
Magic-room: Because items do not take effect Shell bell and Focus-sash can not activate so you can OHKO the ones with Focus-sash and 2KO the ones with Sturdy.
Flinch-moves: They can't use endeavor if they flinch, though this tactic is more risky
Damaging weather: Hail would be best because Aron and Donphan are immune to sand
Fake-out: Similar to the flinching thing, this way they take damage but are guaranteed to flinch
Priority: For Quick attack ones priority will work because you are guaranteed to be faster.
Encore: They are stuck using Endeavor so they can not kill you(With Aron and Donphan you can just switch to a Ground/Ghost Pokemon).
Use another FEAR Pokemon.