Part 1[edit]
Units: 12 Goal: Defeat Lyon
Using a thief, rogue or both unlock the two chests guarded by the gorgons at the bottom of the screen. At the same time split your units in half to assault the bridges into the main chamber. Stay out of range of the dracozombies until you have cleared the bonewalkers from the bridge. Equip your best characters with sacred weapons and send them just in range of the dracozombies to draw them out. Have two units capable of healing nearby and attack them with everything you can to take them out and heal your units. Generally a dracozombie can take out any unit with two hits so be careful. Now proceed forward into the main chamber. Use magic such as purge and bolting to take out the gorgons with shadowshot. Then take a full offensive against Lyon with at least six characters with sacred weapons in range to take him out. Remember that Lyon will not move so a two tile radius will work to stay out of range.
Part 2[edit]
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.