GeoCrawler

The Geocrawler is the Field Boss, guarding the entrance to Aincrad's 56th Floor's Labyrinth. It was considered as a very difficult field boss to defeat, as it took multiple attempts before it was cleared.

Background
It is found in an open field at the end of the 56th Floor, guarding the stairs to the next floor.

The Geocrawler's field is directly opposite of an NPC village, which was used as a safe base of attack for the Front Liners (it has an unbreakable gate). They are separated by a narrow mountain valley which prevents flanking of the boss.

The Front Liners at the 56th Floor were stuck without progress for a week attempting to defeat the Geocrawler.

Appearance and Abilities
The largest known boss revealed so far, it has the height of a one-story house and is many times longer, like a legged armored earthworm or serpent. It has large horns and very prominent armor on its shoulders, back, legs, and front.

According to Asuna, it is practically invincible from the front, since it is impervious to thrust, blunt, and pierce-type attacks. Its main weak point is its soft-looking belly, but due to its massive size and narrow valley terrain, it is extremely difficult to target.

Grand Meaty Meat Plan
The first plan mentioned in Material Edition 01 is the "Grand Meaty Meat Plan," designed by Kirito and Klein, although it was certainly not the first attempt on the Boss. The plan involved baiting the Geocrawler with an A-class meat food ingredient, but the strategy failed like all others with the heroes fleeing back to the unbreakable gates of the NPC village.

Lure into NPC Village Plan
Asuna produces a plan to lure the Geocrawler towards the village, open the gates, and allow the Boss to slaughter the NPC villagers. At the mean time, the players would fight the boss while it is distracted. Kirito rejects the plan and challenges Asuna to a duel, which he wins, so the strategy is not executed.

Put Boss to Sleep Plan
Asuna later encounters an NPC named Ruru, who sings a lullaby, which she realizes is the key to defeating the boss. They then have a musician play the lullaby during the boss battle, the Geocrawler is put to sleep. Then, the boss is easily defeated.

Trivia

 * The Geocrawler is not a Floor Boss, but a Field Boss instead. A Field Boss is a monster guarding the entrance to the Labyrinth, while a Floor Boss guards the entrance to the the next Aincrad Floor.