Fire Emblem: The Sacred Stones/Final Chapter: Sacred Stones

Part 1
Kill the miniboss dragons with Myrrh, then slowly make your way to Lyon. Killing him and his guards can be quite a feat, but nevertheless, it needs to get done.

Part 2
All of the units you bring into this battle, except Eirika, Ephraim, and Seth are likely to die. Stay out of the range of Fomortiis and kill the monsters he summons, then advance on him with everyone who has a legendary weapon or a dragonstone equipped. With a bit of luck, he should go down easily.