The Lemon Jelly and the Royal Lemon Jelly cast the spell Lemon Bone. It deals ??-50 air damage and takes 20-60 agility. The range is at least 7 and casting it takes 4 AP. It also is very likely to be Linear and Line of Sight, so keeping away from line keeps you from being hit.
The exact amount of agility taken appears to be 5 * (level of the Lemon Jelly - 37).