Spoilers
Well, if the issue is that your characters are too low level, you could bring a bulkier party of Warson, Lance, and Milca to try and make it through. Spam her AOE heals, try to focus down Mugen first. Maybe you have some Magic Crests that could be put to use as well. Otherwise, if you're absolutely stuck and at your wits' end, and you understandably don't feel like restarting the whole FE, you could always turn to RPG Maker save editors online.
