I just cant beat my friend's combusken cuz im grass type.
Rock Slide (Coverage)
Leaf Blade (STAB)
Arial Ace (Infinite Acc)
Double Team (Make it Harder for them to retaliate)
And yes, it is quite difficult to beat Fire with Grass. I would use another Pokemon against Combusken if I was you.
For me it would be:
swords dance- raises your attack sharply
cut-alright damage and a very useful HM
solarbeam-most powerful grass-type move I know
and giga drain-covers when HP is low