Dirge is located in the far west region of the map. There are any number of ways to get there, the easiest being to cross over to the northern continent immediately and begin heading due west. Follow the sword's light until you come to a ravine. Alternatively, You can choose to go through the southern continent and cross over on the natural bridge on the far left side. You can also shortcut through the forest just southwest of the temple. Going this way is longer but you also will find some more save points and white tailed lizards. Dirge is in the cavern at the end of the ravine.
Dirge, the "Sand Tiger", lies waiting under the sand in this cave. His design is similar to the electric eel you fought earlier, but defeating him is much different. Like the last battle, Agro is essential to victory, so don't get off him unless told to do so.
Once Dirge awakens, lock your vision onto him and ride Agro around the sandy arena. He'll begin chasing you right away. It is of the utmost importance that you remain on horseback at all times, for Dirge will outrun and crush you the moment you try to run on foot. If you are thrown off of Agro, run for one of the stone pillars scattered throughout the cave - Dirge cannot cause you any harm while standing on the rock - and call for Agro. Equip the bow and let the hero turn around to aim at the colossus (when locked on he'll turn automatically). Dirge will lift his head out of the sand and eventually open his eyes. Take over control of the aim once the eyes appear and fire an arrow at one of them. This isn't easy - it requires a good aim since your foe's face does move quite a bit whilst pursuing you. If you hesitate too long, Dirge will disappear under the sand again before lunging at you from below. Dirge will cry out once you hit, and continue twisting blindly forward.
Keep an eye on him until he finally crashes into a wall and starts writhing around, then ride Agro towards his back and climb up. This colossus has two weak points along his back, with the second located a little higher up. No matter which one you choose to tackle, he'll usually remain stunned long enough for you to finish off that symbol before burrowing again. When he does, simply jump off, mount Agro, and repeat the process a second (and final) time.
- This boss is no different on Normal or Hard Modes
Time Attack Strategy
This time attack is pretty straight forward. Mount Agro immediately after the opening cutscene, and start galloping away from him. You should be able to get Dirge to chase you right away. If you can hit him this first time, you'll make your life a lot easier for the second. You may want to take the vital further up his back first, but either way you should have plenty of time to beat Dirge in hard or normal mode.