Infernax

This can be done towards the end of the garrison soldier's quest chain while working toward the Ultimate Good ending. After gaining enough good morality to be offered a quest from each soldier in each of the three town garrisons (except for Tancred, for now), complete every quest that they give you. Talking to Tancred afterward will initiate the quest 'Destroy the Cult', where you will be invading the cultists hideouts.

Meet Tancred and his men at the Tohan Passage cultist hideout, and say Yes when he asks if you're ready. Tancred and the two soldiers will enter the hideout. Instead of following them in, zone out of the current screen you are in (drop down and enter Arkos, for example, anything that transitions to a new area). Return and enter the hideout, where Tancred will have some words for you.

Next you will be attacking the hideout at Mount Kastka. Repeat the same process, talk to Tancred, say Yes, don't enter the hideout but leave the current area, initiating a screen transition. Return to the hideout, enter and the achievement will pop after a short scene.

As a side note, the third hideout is not part of this because Tancred makes you go in first for that one.

Here's a written walkthrough:

1. Work on the ultimate good playthrough, helping all the soldiers in the towns.
2. Once you're "good" enough, the guard in the garrison in Kastski will tell you to help Tancred.
3. Go to the right of Darsov, and help Tancred with an attack. He will ask you which city to fortify.
- I fortified Kastski, and that will unlock this achievement.
4. Go find Tancred in Darsov's garrison, and an assault will take place on Darsov.
5. After the assault is over, talk to Tancred again, starting a mission to attack the evil bases.
6. At the first base above Arkov, start the attack, but don't enter. Travel to another screen, then back, then enter. The assault will be over, and you can talk to Tancred now.
7. Go to Mount Kastka to the second evil site. Do the same thing, start the attack, but leave, and return.
8. When you enter, talk to Tancred. Leave, and the achievement is yours!