The entrance to the Fire Giant's Dungeon is in level 50 wilderness. Walk down the stairs near the red exclamation mark on the map, located west of the Mage Arena. If you are willing to take the risk of entering the wilderness, this is a good place to train for rangers and mages; due to the 5 or so Fire giants at the end of the dungeon. Watch out for the aggressive spiders in the room adjacent to them, if they hit you they will drain half of your prayer, so come prepared. All the monsters here are aggressive, thus good for many types of training.

Chaos Dwarf (Level 48) Fire Giant (Level 86) Hill Giant (Level 28) Shadow Spider (Level 49)