Since this question has been answered and will soon become archived among the thousands of other questions, I might as well list all the grass types that others came up with.
Stuff that is known to work
Mega Venusaur - fire hidden power
Breloom - mach punch
Tapu Bulu - superpower
Cradily - earthquake
Serperior - fire hidden power
Stuff that might work
Tangrowth - fire hidden power and earthquake
Chesnaught - drain punch
Virizion - sacred sword or focus blast
Dhelmise - earthquake
Roserade - fire hidden power
If you came up with something that isn't listed here, that's probably because I decided that it had too many weaknesses to be good (a common problem with grass). This is the case with Shaymin, Celebi, and Silvally. I didn't list mow Rotom because it doesn't learn good moves that are super effective against steel.