Use a Sludge Wave Sheer Force Life Orb Nidoking or Landorus-I (the genie one).
I recommend using the standard Special Nidoking set (with Fire Blast in Thunderbolt's slot) to roast Ferrothorn, a major grass staple:
Nidoking @ Life Orb
Ability: Sheer Force
EVs: 252 SAtk / 4 SDef / 252 Spe
- Sludge Wave / Thunderbolt
- Earth Power
- Fire Blast
- Ice Beam
252 SpA Life Orb Sheer Force Nidoking Fire Blast vs. 252 HP / 168 SpD Ferrothorn: 468-551 (132.9 - 156.5%) -- guaranteed OHKO
252 SpA Life Orb Sheer Force Nidoking Earth Power vs. 252 HP / 0 SpD Gastrodon: 220-261 (51.6 - 61.2%) -- 94.5% chance to 2HKO after Leftovers recovery (Can't switch into you)
252 SpA Life Orb Sheer Force Nidoking Earth Power vs. 232 HP / 4 SpD Mega Venusaur: 160-188 (44.5 - 52.3%) -- 22.3% chance to 2HKO
It will take a lot to wear down Venusaur either way.
You can then core with Gastrodon to have a reliable water check as well as a semi-ice check. I'd recommend something to fight Weavile in Ice, however. Scarf Excadrill is a good check, or even Sand Rush if you are playing a Sand mono Ground (one of the most common kind of MonoGround).
Source: Monotype experience in all 17 types