So what if FE1 only had bow-using units? No, I didn’t just act on my first impulse when I saw a similar recent Serenes thread and threw together this whole project in about an hour! What are you talking about? 
Fire Emblem 1, but every unit in the game, enemies and playable units, now are types of archers. Horsemen are treated as promoted units, so all promoted units are converted to Snipers or Hunters. If a promoted enemy becomes a Horseman, he gets +10 Level to compensate for the fact that FE1 doesn’t considered them promoted normally. Normies have a chance to randomly spawn as Horsemen instead of Archers/Hunters too. All shops now sell bows instead of other weapons. Chests are untouched.Oh yeah, and Hunters can promote to Horsemen. You can also deploy less than the required amount of units if you’re so inclined.
I can’t see any reason why you’d be unable to beat the game this way. “How do I kill Gharnef?” you may ask? Well, he’s an archer too, so just fill him full of arrow like anyone else and net yourself another Parthia.
Also included is a Hard Mode in case using only archers is too easy for you. 
Get here: https://drive.google.com/file/d/1-NPWekhy9ON9TXTAo95HdnfcBTgqJcWH/view?usp=sharing
Edit: Apply to a normal Japanese ROM 'cause those are always a safe bet.
Now I need to get back to the other project I should’ve been working on instead of this. 