Crawmerax was created to be the most difficult boss in Borderlands. While this is inherently true (he is a level 64 enemy while the player can only reach level 61), there are several known exploits to make the fight easier for solo players.
Crawmerax is essentially a fully grown Larva Crab Worm, they are seen in Lost Cave and Trash Coast areas in the original Borderlands game. Crawmerax does not fight alone, however, he has an army of 3 additional new Crab Worm types not seen before: the Green Craw Worm, the Craw Maggot, and the Armored Craw Worm. These enemies will aid Crawmerax in the fight and can often prove to be more of a nuisance than Crawmerax himself.
Killing Crawmerax is the primary objective in the quest entitled "You. Will. Die.", which is unlocked on the T-Bone Junction bounty board via the completion of the main questline in The Secret Armory of General Knoxx. However, Crawmerax is a repeatable boss fight, and players are encouraged to defeat him many times over for a chance at the best loot the game has to offer. Killing him is considered to be the "easiest" way to discover a "Pearlescent" gun, the new rarity level after Legendary (orange) added through the General Knoxx DLC.
Crawmerax may be a female Crab Worm seeing as there are multiple Crab Worms (and maggots) in the staging area, Crawmerax's species may even be asexual (reproduce by themselves) seeing as Crawmerax is the last fully grown Crab Worm due to hibernation. It is logical that Crawmerax could be an asexual creature.
Crawmerax has three brand new types of worms, as mentioned before, there is the green craw worm, the craw maggot and the armoured craw worm.
Green Craw Worm
They have the most health out of the other two worms. They look like a crab worm without any armored shell/plating. They spit an acidic substance that may be a lot like a Komodo Dragon's saliva (poisonous due to the fact they have 'rotting' gums).
Their weakness is fire, they will burn and die after a couple clips of incendiary (fire) weapons.
Armoured Craw Worm
These guys hit the hardest and have the most armor. They look almost identical to the Larva Crab Worm in Lost Cave, but they're a bit bigger and they are (always) level 61. They spit very rarely because their melee is their real power. A caustic (acid) weapon is best used against these craw worms, a high leveled Pestilent Defiler (e.g. 54 picked up from play through 2.5) can kill these guys in a couple shots.
These craw worms are the middle ground, they hit hard but not quite as hard as the Armored craw worm and they have a good deal of health but not quite as much as a Green craw worm. If they were left for a long enough time (it is thought) they would grow into Crawmerax (fully grown Crab worm).
These guys hate electricity (shock) damage, just like their mother/father. A good Eridian lighting storm (e.g. level 51) can take them out in three or four shots.