I'm playing a level 24 warrior on normal mode. I couldn't beat Kalagru even after finishing the Ark of Lothasan (the agreeable way) with a bunch of great gear (Adamantine sword, etc.) because I'd invested 5 points (without counting bonuses) in each of the three good conversation traits (personality, etc.). So, my HP and DPS weren't high enough, he'd always subtract damage far too fast for Hirge to be allowed to use another Circle of Restoration (I couldn't use them myself).
Took me awhile to realize what I needed to do. I went to New Garand, and I paid the Warrior's Guild person to reset my trait point and skill point selections. Then, I got the loreseeker in the Great Library to teach me to use scrolls that require mana even though I'm not a wizard.
I invested most of my trait points in strength (I got the base all the way up to 10), and a bunch in endurance, and only as much as needed in the others (2 personality allows you to get the +10 DPS "build up each swing" bonus against an enemy you're hitting multiple times that you learn in... Nivarian?... in the Warrior's Guild). I had only as much agility as needed for the Fury skill.
Then, for skills, I bought up to the second level of the "use magical scrolls" trait (to reduce HP sacrifice, since each HP point and damage point is precious against Kalagru), and I specifically did NOT invest any points in the skill that increases your daily full heal button occurrences, or anything in Flurry (because Kalagru is one enemy, and that skill is all about hitting multiple enemies). I instead maxed out the skill where you spin in a circle (Slash?), the one where you cut through enemies, the one for shielding, etc. Basically anything useful for Kalagru. Maxed out my Fury skill, too.
Then I bought 40 Circle of Restoration scrolls (overkill... I think I used maybe 10 or 15, I thought it would be harder based on my past experience) from the lady in the Great Library. And then I sailed from New Garand to New Anthur, went up through the Solligan Strip, and tried again!
Basically, in the fight, both my characters (main and Hirge) could now interchangeably use the Circle of Restoration scrolls, and if he ever decided to focus on Hirge (because I would maybe use a skill to slash through him that would carry me a bit away), I could use a scroll to bring her back, and then she could use one to regenerate my health. After about two or three tries, I got him!