Jellyfish are very dangerous ranged units, primarily due to their ability (to decimate half an army with endless arrows). However, firstly, this ability to decimate half an army only works on living units (similar to the powerful gargoyle's gaze from Heroes of Might and Magic III), and secondly, if a strong melee unit from the enemy reaches the jellyfish, their lack of a melee penalty won't save them – they will fall too quickly. Otherwise, they are one of the best offensive units at their level, although they won't survive against ballistae.