- Charging Attack
Ranged Strategy (easy mode)
To avoid a frontal approach, go to the area just before the depths where you defeated the butcher. If you destroy crates around that area, you will see a small pit by the wall. Fall down the pit into a large pile of bones. From here you can use ranged attacks from the ledge to pick at the Giant Undead Rat at your leisure.
A long range melee weapon such as the halberd or spear are preferred. Before entering the mist in the Depths take a right into a narrow corridor. Be careful not to fall in the area where water is rushing as it's an open trap. Take your first right, go to the end of the next corridor and take another right. You will be facing the Giant Undead Rat's room. Move into the room and then immediately roll back to where you were. If you time this correctly you should be able to get one hit on the Giant Rat as he charges by you. Lather, rinse, repeat.