This boss erupts out bullets even out of its Range of Sight. These can deal substantial damage to the characters, especially considering it's backed up by two of each of the normal enemies from the stage. The boss is weak to Physical attacks, and magic is almost completely ineffective. If the character attacking the boss is a melee, it should most likely anticipate where the bullet will land and dish out damage by dodging. Otherwise, ranged characters should not interfere with the boss's limited range of its attack. A Boxer with a Mach Claw 5 with a Critical's Card and a Berserk Card, combined with large amounts of STR, can easily destroy the boss. If a Physical weapon is not available, a Freeze type weapon will suffice.
If the Blue, Orange, and/or Yellow Roundhead Stickmen get in the way of the boss, have the three other stickmen use their respective types of weapons against them, while keeping one to attack the boss as a distraction. Once the other enemies are out, let the other characters switch back to their Physical or Freeze type weapons.