Use the Ground Gargoyle in Level 2 to push aside the statues in the water area. The one pictured here leads to a hidden part of the stage. Follow it to the end to reach Ovnunu, an eyeball boss.

Beat Ovnunu, and you will receive the part of the Fire Crest called the Buster. This will allow you to break through certain blocks.

    

Go to Level 3 and follow the main path through the forest all the way to the end. You'll find the boss, Flame Lord. Beat him for the part of the Fire Crest called the Tornado. You can use this to make platforms to travel upwards (only useful before getting the Aerial Gargoyle).

    

Go all the way to the end of Level 4 (don't fall into the whirlwind, keep going past it). You'll reach the Flier boss at the very end. Defeat the Flier for the part of the Fire Crest called the Claw. The Claw allows you to shoot sticky substance onto vertical surfaces, so that you can safely cling to walls that have spikes on them. (Honestly, it's not all that useful.)

    

Go to Level 6 and follow the lower path all the way to the end. You'll reach a white wolf boss named Grewon. Defeat Grewon for the part of the Fire Crest called Demon Fire, the most powerful weapon in the game. (Be prepared for a long drawn-out fight with Grewon. Make sure to take some Ginseng potions with you to restore HP during the battle. You'll probably need it.)