Keep Your Blades Sharp Achievement
Keep Your Blades Sharp15
Stop the Necromancer and let the kingdom begin to heal.
Complete Mission 45.
This final mission is a little different than any of the others, where you would normally want a very high-level ally. Instead, bring someone completely worthless with you. The final boss of this mission will turn your ally against you, so you want someone you can defeat easily.
Otherwise, the entirety of the 45 Missions are simply a hack and slash/crossbow shooting gallery. Kill all the enemies in a given area to proceed, rinse and repeat until you finish the levels.