Welcome to Warlords! When I ran this it was not on a stock 90 character but a geared level 90 priest during the lead-up quests for the expansion pack. You will find Gar'mik inside of Okril'lon Hold, third floor. The entrance is located at 41,11.
Gar'mak was a simple kill. You will go inside and have to battle a few mobs, there is nothing that can kill you on the first floor. The second floor will only one one or two mobs depending on who went before you. Kill them - BUT DO NOT GO through the arches and outside where you will find plenty of mobs to kill. Instead, you will see a winding staircase to the third floor. Climb to the top and there is Gar'mak and a single mob there. Once you start the attack, Thrall will show up and help you finish him off.
A tip for priests and any one with a pet. Put your Mindbender (aka Shadowfiend) or pet on Gar'mak, hit your fade to drop aggro, then kill the mob, then kill Gar'mak.
Bladestorm is a primary attack, but unlike some of the ogres outside, Gar'Mak will slowly follow you as you back away during this attack. It is still easy to kite him.
I am sure this will be harder for a stock level 90 leveling once the expansion pack is released, but easy for a geared 90 coming in from Mists.
Get clear quick, because Gar'mak respawns fast.
Quick escape tip. If you go down to the second floor, inch just outside the door, and from there you can mount a flying mount and escape without having to fight your way out. Kite the Ironmarch Pillager inside, kill him and they mount in the doorway.