ok i cant beat gharnef o_ô how?????????
You can't beat him in the desert. Imhulu nullifies all damage from weapons other than Starlight. So just wall him with a strong unit that is fast and has plenty of HP (Barst, Oguma, etc) or a RES tank (coughLenacough). Hold him at the bridge for a few turns, then run like hell. He'll just leave after enough time has passed.
FE1: Leaves pretty early.
FE3B1: Leaves...wtfearly. Not a threat lol, when you get to that map in FE3b2 Astram's team is more threatening than he would've been if he were there lol
In Shadow Dragon, this time, he'll actually catch up to you at a more reasonable time. Definitely use Lena or a throwaway unit to bait him, though; pure water also helps; she'll take like, 1 dmg; 2, if she's doubled, around that time. No need to risk good units like Barst and Oguma, especially since you'll likely need them for the Dragon Knights if you're not using Merric.