The Infected Bat has four unique attack moves in this game. The following stats are derived from the biohazard 0 KAITAISHINSHO guide. With the exception of "Body blow", the Infected Bat can begin any attack at any distance.
It is best to use Billy in this fight on account of his higher stamina and greater overall accuracy. This boss can prove rather frustrating to fight for two reasons: the giant bat's high mobility and the presence of smaller bats. The Shotgun is likely your best bet on account of its spread. The Grenade Launcher, while dealing heavy damage per hit, lacks the range and speed to effectively hit the boss.
The sheer mobility means that you have to aim and then fire immediately. All attack, bar the body blow, deal moderate to high damage and can amount to an astonishingly quick finish. Have a steady supply of healing items ready and do not hesitate to use them.