GeoCrawler

The GeoCrawler (ジオクロウラー) was 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
The GeoCrawler was 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 Non-Player Character (NPC) village, which was used as a safe base of attack for the Clearers, due to its unbreakable gate. They were 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 GeoCrawler is the largest known boss revealed so far, as it is the height of a one-story house and is many times longer. It looks 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 slash, 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
Among the first plans developed for fighting the boss was the "Grand Meaty Meat Plan," designed by Kirito and Klein, which involved baiting the GeoCrawler with an A-class meat. However, like the other ideas they had created, the strategy failed, forcing the heroes to flee back to the unbreakable gates of the NPC village.

Lure into NPC Village Plan
Asuna produced a plan to lure the GeoCrawler towards the village, open the gates, and allow the Boss to slaughter the NPC villagers. At the meantime, the players would fight the boss while it was distracted. Kirito rejected the plan and challenged Asuna to a duel to decide if the plan would be followed. As Kirito won the duel, the strategy was not executed.

Put Boss to Sleep Plan
Asuna later encountered an NPC named Ruru. When Ruru sang a lullaby, Asuna realized it was the key to defeating the boss. They then had a player who had mastered the Musical Instruments skill play the lullaby during the boss battle, immediately putting the GeoCrawler to sleep and allowing the Clearers to easily defeat it.