In the 2D versions, it is not a particularly threatening enemy for the stage of the game it appears in. In these versions, Beam will only take of 1/10 of a character's max HP each hit, which can easily be remedied with by using Cura or Curaga.
In the 3D remakes, Beam deals about 1700-2200 damage or more, quite independently of the target's max HP. This makes it a much more 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. When hit by a Lightning attack in the 2D versions, the Beam Cannon will start hitting itself and its own party members with Beam instead.
They are much tougher in the 3D remakes; a single multi-target Thundaga will not be enough to eliminate them. Focus fire on the Beam Cannons; Slow may also be advisable.