Egil must survive. He can be injured in fights, he dosen't die this way, but there are certain story events possibly killing him, so be aware of the following situations:
When you reach Skogr in chapter 2, right before the fight you can decide if you want Alette and Egil fight with you. If you send them away you're save, if you let them fight, a dredge will threaten Alette after the fight you have to chose in that situation first: "Shoot it with an arrow" followed by: "Stop thinking! Shoot the dredge!" to save Egil, if you take another option, Egil will die here.
At the end of chapter 2, in Frostvellr, if you decide to get into the town by ambushing the supply cart, you've to win the fight, otherwise Egil will die.
In chapter 4 don't recruit Onef.
In chapter 5, when you're in Eintartoft, on the second day you have to fight personally on the bridge, don't clear the bridge of dead bodies, as Egil will die this way. Make sure you fight and win on the second day.
The achievement will pop at the end of the game.