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!