Well... Of course ideally you wouldn't want to take on a Fire-type with a Grass Pokemon like her. But if you have to, I'd recommend fighting indirectly as much as you can. Use status effects like poison or paralysis, cover the enemy with a Leech Seed, use moves that lower their accuracy and strength if you have any, that sort of thing. And teach her some non-Grass moves if you can. A Water or Rock type attack from a Grass-type Pokemon can catch people off guard pretty nicely.