To kill Marr easily, develop a good male or female ranger, then just range him. From the start, make sure you have a bow that can do at least 90 to 120 damage. If it has two sockets open, put any Whetstone in it and any type of Misty Globe. If you have a lot of points on Dexterity and Strength, your damage should now be about 250 to 400 points. Make sure that your Cold Arrow is at least level 3, your Mana is at least 250, and your Critical Hit is at level 5, 6, or 7. When the match starts, get on a good camera angle, put on your Critical Hit, and range him until he dies with your Cold Arrow. When you start your ranging, he will try to block your arrows, but unlike the normal arrows the Cold Arrows will always hit him.
This is directed specifically to classes that can summon an ally and can use a decent projectile (magic recommended because it does not run out). Summon the ally immediately at the start of the battle and turn on the map so that you can see the Planar Portal Arc located to the south. In the short time it takes for him to kill your ally, run to the arcs and get behind it. Because the middle of the arc is open, you can shoot him without getting hit once. Be careful, as he will attempt to run around and catch you. Just keep shooting him until he dies.