Beam deals about 1700-2200 damage or more, independently of the target's max HP. This makes it a dangerous enemy, especially when combined with its high speed.
Using Rydia to summon Leviathan, or having Rydia or Fusoya cast Thundaga on all of them is more than enough to take them out. A single multi-target Thundaga will not be enough to eliminate them. Focus fire on the Beam Cannons; Slow may also be advisable.