There is a Break Off located on Glas Ghaibhleann's right wing, which gives the Torn Glas Ghaibhleann Feather. It takes two Chain Hooks at the same time to activate a break hit, and it must be done twice to get the break off. Glas Ghaibhleann will tear his own wing off, and because of this a random player in the party will be credited with the break off.
A single Ballista is located in this battle. Ammo for the ballista can be found on the right side of the map at the start, and at the far left corner near the blue warp (to get to the second floor). This ballista can hold up to three spears. It takes two consecutive shots from the ballista to stun Glas Ghaibhleann. Glas Ghaibhleann will drop to his knee for a few seconds when stunned. This is the perfect opportunity to get the break off.
A unique secondary weapon called Judgement of the Goddess can be found on a rack on the top left section of the map (it glows light blue). This will replace you current secondary weapon with two Goddess Spears. If enough players take the Goddess Spears, they will automatically refresh after a few minutes. The Goddess Spear can be used to stun Glas Ghaibhleann if they are thrown into his foot. Glas Ghaibhleann will flail harmlessly while stunned by the Goddess Spear.