This boss is encountered at the top of the Ipsilon Mountains. As the group approach the peak, the Grilgan spots them and engages in battle.
Grilgan has a few really nasty attacks. One, Down Burst, can be a real pain. It inflicts several hundred damage to all three of your characters; if you are unlucky enough to have him do this a couple times in a row, it will likely be deadly for you. Poison Claw is also pretty bad. It does big physical damage -- well over a hundred for Kaim and Seth in the front row, but will then do even more at the end of your turn -- and since he goes after you, that damage is essentially applied directly to the original attack damage. The damage total for the first turn alone will probably be around 220. And even Grilgan's physical attacks aren't very pleasant... The key here is to just damage Grilgan as much as you can, as quickly as possible. The longer this fight goes on, the harder it will be to win.
So, right off the bat have Jansen cast Flare, and have Seth toss a Flare Bomb. Equip Kaim with a Bruiser Ring, and try your best to land perfects so that you do more damage. Don't bother wasting your turns healing. Just let the characters die. Jansen, though a great damage dealer is not worth the effort to bring back, and the immortals will get themselves up after a few turns. You just have to be a little bit lucky and hope Grilgan doesn't get too aggressive in the mean time. On the bright side he doesn't have TOO much HP. With three characters you should be hitting for 300-400 damage per turn, and even with two you should be getting 200-300.
Note though, that a lot of people claim this fight is really hard. If you absolutely can't seem to win, try leveling up to 15. That will significantly improve your odds. Also, make sure that you have Crisis Attack Boost (learned from the Berserker Necklace accessory) on both Kaim and Seth. There's a very good chance that it will make the difference between winning and losing, in the end. Instead of just equipping a Bruiser Ring, try using a Flare Ring it will give a nice damage boost. To make sure you will not be affected by poison, you can use the accessory Antidote Brooch to learn the Anti-Poison skill which you can obtain in Uhra earlier by completing the Errand: The Soldier's Helm.