Xantan the Foul
To get to Xantan the Foul, you must go to the dank cave northwest of Neopia City. There are four levels in the dank cave. Xantan is the fourth level. The fourth level is the hardest of all the other dank cave levels. Xantan is in a place which is a maze that you must travel in circles until you get to him.
To get to Kreai, travel south of Neopia City until you reach the Jungle Ruins. Once you enter the ruins, head to the east until you reach a staircase. Go down the staircase. Head east but do not take the staircase that you see while traveling. Instead, keep traveling east until you can't anymore. Afterwards, travel all the way to the south and ignore the chambers on the east and the west. Once you are at the southern most point, head east until you are at the exit of the narrow path. Kreai will be at the north in this area.
Gors the Mighty
After you kill Kreai, take the teleporter. Remember the staircase I told you not to take? Now that you killed Kreai, take the staircase down. Once you are on the second basement level of the ruins, head south till you see a second opening. Head east until you reach a staircase. Do not take the staircase. Head north and keep moving until you get to the most western place on the map. There will be a narrow path. Take the path. The path to Gors the Mighty will be traveling through a labyrinth of teleporters (ugh). Gors will be in an underground garden. Once you are in the underground garden, just keep heading north.
After you killed Gors, take the teleporter back to the second basement level. Remember that staircase? Take it now. Now, the third, and last (finally), basement level is the largest of all the places in Ancient Neopia. You must travel for a long time before getting there. Rollay is on an island with a path, of course, in the middle of a large lake.
Archmagus of Roo
The archamgus is in the second level of the Temple of Roo. Before encountering him, you must find six keys in a particular order that are held by his servants. The first key is held in the room in front of you when you are in the narrow passage. After that (in this order) go to the northeast room, the southern most room, the northwest room, the eastern most room, the room in the center, then finally, the southeast room where the archmagus is hiding.
Guardian of Shock Magic
This monster is in the Mountain Fortress. When you first enter, this monster will be in the first chamber in the west. Enter the chamber and head south.
Guardian of Fire Magic
This monster is in the Mountain Fortress. It will be in the second chamber to the west. Enter the chamber and head south.
Guardian of Ice Magic
This monster is in the Mountain Fortress. Head all the way south upon entrance and head west.
Guardian of Life Magic
This monster is in the Mountain Fortress. It will be in the second chamber to the east. Enter the chamber and head south.
Guardian of Spectral Magic
This monster is in the Mountain Fortress. It will be in the first chamber to the east. In the chamber, go to the middle cubicle and head south.
Faleinn is the leader of Kal Panning, city of the dead. When you first fight her, there will be an option called "Show the Keladrian Medallian to Faleinn". Show it to her and she will be instantly defeated. You get the Keladrian Medallian by repairing to in Sunny Town, which was formerly the Rusty Medallian. You get the Rusty Medallian by defeating Rollay Scaleback.
Jahbal is the last boss in Normal difficulty. He can be found in the third floor in the Palace of the Two Rings. On the third floor, there will be no monsters there except Jahbal.
Mastermind is the last boss in Evil difficulty. He cannot be found in Normal difficulty. Mastermind will appear at the moment you click on "Click here to continue" after killing Jahbal.
Xantan Reborn is the last boss in Insane difficulty. Remember when you killed Xantan the Foul, his last words were "I will return"? Well, he did return out for revenge. He appears at the moment you kill Mastermind.