This achievement is called "Eagle Savior" 20G
Defeat Agandaûr without the aid of Beleram.
You can earn this during the final fight with Agandaûr (there are two fights), a message will pop up saying Beleram is seriously injured, but will assist you if needed. You will now have the ability to summon him as you have previously in the game. Simply DO NOT summon him.
The message popped up for me when Agandaûr had less than half his health remaining, and since I like Beleram a lot, I didn't want to put him in danger (didn't know about this achievement), and was surprised to unlock this.
It unlocks during the cinematic after defeating Agandaûr.