The Demonwar Crypt is a good place for mages. To reach it, enter the Edron Hero Cave and go into the portal here. When going through the teleporter you will appear in a room with NPC A Dark Priestess. From there you can take stairs up to the Demonwar Crypt. Be prepared to fight 4-6 Shaburak Demons at once, so take with you a few Energy Rings to reduce waste. Bring a lot of Stone Shower Runes\Fireball Runes and use Magma Set\Terra Set for hunting Shaburaks\Askaraks. Also bring a couple of Backpacks to collect the huge amounts of gold they drop.
You can ask any Guide NPC after the Demon War and he will inform you of the power balance between the two races of Demons. Note: The Princes and Lords only spawn when this balance is disturbed, and Guide NPCs can tell you whether or not this is the case. To disturb the balance, kill a lot of Demons of just one side.
Tips and Tricks
Use Poison Bomb Runes for trapping Shaburak Demons and Fire Bomb Runes for Askarak Demons. After the ladder to the 2d floor there are 5-6 demons at once, so fields will help you to win some seconds for casting Magic Walls or Wild Growth.