`HP Grass on Hippowdon: 55.48% - 65.24% (420 HP, 180 SpDef)
A little bit more problematic, but the Hippo is 2HKO'd if he switches into stealth rock and take two HP Grasses.
Flamethrower on Bronzong (mixed): 53.55% - 63.02% (338 HP, 317 SpDef)
This calculation was done on a sassy Bronzong with 80 SpDef EVs. These tend to be the Bronzongs with the most special defense. Anyways, Flamethrower 2HKOs with stealth rock, and has a decent chance of doing so even without it.
You could also go HP Ground over Earthquake, which could really suprise some stuff, just a thought, good idea, it is looking more like its colegue Magmortar now.