Another unique encounter, mainly because of the fact, that the main enemy (the boss) is not the most dangerous foe (as it poses literally zero threat), but the high number of orcs that will accompany the boss. Garkula can be taken down from a single, lucky attack from your warrior, and even if that does not happen her attacks are average at least, which means that you have to take care of the rest of the orcs before you focus her.
The easiest way to divert some of the enemy attention off your party is to use charm arrows and spells - during this encounter you should have at least a couple of those projectiles available. The best targets for those are the Orc Elite Rangers, which will normally rain elemental arrows on your party (especially if you are playing on the tactician mode), as well as on the shaman, hidden in the nearby foliage.
As soon as you charm at least two enemy orcs, focus your attacks on Garkula, to get rid of her quickly - the best class for that job is a warrior, with the Flurry ability, which can take the boss in one hit, but other attacks, as well as spells can be used as well. When she is down, start eliminating the remaining orcs one by one - note that a few more of them will come from the north.