The second scenario which awards the achievement, is a pretty straightforward war between two alliances. The Gilden, although technically part of the "team", will stay behind their Gate, unless you can trick them into participating. Predictably, your allies are not good for much other than trading a couple of towns with the enemy. I bypassed them at a certain point, leaving two enemy cities for them to conquer (mind that I wiped out any enemy army in the vicinity), but they never actually took these cities until the end.
The Dance with Dragons research is very important here. Once you discover and occupy the Dragon's lair, you will have access to a Storm Dragon. This will allow you to defeat the elemental boss in the North and in turn gain access to a very powerful blade, which grants Blizzard every 5 turns. The Blizzard/Dragon combo is unbeatable. With this, you can overrun enemy cities in 1-3 turns in order to reach an opening in the mountains and make your way to the NE corner of the map. That is where your objective lies: the Forge of the Overlords
The final fight is pretty confusing. Once inside the Forge, Relias will face an army of demons all alone. Like much in this game, this fight is as unintuitive as it gets. The dialog window might mention that you are unstoppable now, but apart from the HP, your new god-like abilities are well hidden. I had to try the battle several times (after a point the fight just froze because I was not doing what I was supposed to, even though there was no actual hint). Finally, I discovered that my Sovereign had gained new abilities under Skills, not Spells, although they are clearly magical in nature. The Mastery spells are very powerful and once discovered should make the battle really easy. The Fire spell is a room-wide AoE, the Life spell obviously heals Relias, Water spell is, in effect, a Blizzard, Air is an AoE stun and Earth has knockback effect. The only challenge here was actually FINDING the damn things in the UI.