There isn't a guide to my knowledge, but you can watch
this video. The first NPC is found by using Quickmove>Ship>Mysterious Path 3> 2 or 3 maps to the Left until you get to the town with Ralph the Wanderer. The second NPC needed for the quest is found by using Quickmove>Ship> Ulu City Entrance and he's near the top right hand corner. He'll give you a bunch of quests that require you to kill mobs and collect items from them. Then from the Ulu City Entrance, once you've done all the other quests, head all the way to the right until you can't go right anymore. There'll be a bunch of mobs that look like fat trees. in the middle of the map there's a yellow circle portal that doesn't show up on the mini map, enter it, scale the building avoiding the hazards, enter the portal, there's a branch in the middle of the tree that you need to ATK regularly which will summon Krexel. Defeat him and then go finish the quest. For more clarification on how to actually get to Krexel (once you've done the quests) use
this video to help you. Hope this helps.