After completing almost all of Defiance's Bay quests (sadly, the place isn't as big as Baldur's Gate and Athkatla), I decided to go back to Caed Nua and explore the underground. Reached and completed floor 5 without any problems (party level = 8). My Erék is almost impossible to kill, and there is no need to use spells or potions to protect him further (with 93 Deflection, 80+ Fortitude and 65+ Reflex/Will, almost no one is able to hit him, and I can even cast some spells which requires a Fortitude check on top of him in all impunity).
I also discovered a tad late that we can flank enemies, which gives them a malus of -10 Deflection. So, basically, I send Erék on the front lines, and I ask the snow fox to go around them in order to position itself for flanking. It's very effective.