Crawmerax is the secret final boss who you face during the mission You. Will. Die.,which is unlocked from the bounty board after completing the main story. If your character is not strong enough to survive Crawmerax's devastating attacks, there is an easy method to defeating him.
After the elevator brings you up, you will see Crawmerax appear in front of you. Run to the left side of the area and drop down onto a small ledge and make your way up to the corner against the high wall and the step up to the main area. From here, Crawmerax and his minions won't be able to attack you. Instead, they will all turn to face you and roar, allowing you to easily target the bright purple weak spots and kill him. It is highly recommended to have a powerful sniper rifle and a method of regenerating ammunition, but it is absolutely necessary to have a rocket launcher with you because the weak point on Crawmerax's back won't be visible from this angle. You will have to aim your rockets at the spikes on his shell so that the weak spot is caught in the explosion.
This is also an excellent method to quickly level since killing Crawmerax's minions earns you a considerable amount of experience.