Cottonee(evolves into Whimsicott by sun stone) is a good choice for grass type, plus got that nice secondary fairy typing(taking care of dark type and fighting type), but it is a Mystery Egg content, as Mystery Egg is predetermined in each save files, you just have to have the luck to come across a Cottonee in early game~It is obtainable as wild encounter very late in game tho~Other decent grass types are Roserade(budew line) and Lilligant(from Petilil)~Lililgant is pure grass and learns a very powerful boosting move called Quiver Dance(increase three stats in a row, special attack and defense, and speed) in high level, allows you to handle the special attackers~Fossil mon Cradily also functions as a wall and got extra rock typing to cover some of grass weaknesses(like flying and poison)~Last one is Shroomish(evolves into Breloom), Breloom is fighting type, and ability poison heal recovers some HPs each turn when it is poisoned, not damaging but heal, very handy with Toxic Orb(unfortunately also as Mystery Egg)~