You must be at least Level 25 to open or receive this quest. See Elyos 25 for more quests that can be started at this same level.
† All quests reward XP but NCSoft is fond of changing the amounts frequently, to the point that it is simply not wise to try to track the exact amount in a wiki.
Flower Wall - 148.7, 311.1, 228.1
Lightning Wall - 367.7, 288.5, 218.6
Wave Wall - 212.1, 395.1, 158.3
Wind Wall - 304.1, 116.9, 155.4
Fire Wall - 163.8, 144.6, 153.3
Patch 1.9 added a repeatable quest, The Shugo Menace. Each repeat of the quest will reward some XP and ONE of the four Wall Fragments. That should make this quest much easier to finish for those that cannot get consistent groups.