Thrashers can only be killed by either shooting all the spores off their body, shooting a charged shot of the KT-4 at them, or using the Skull of Nan Sapwe's vaporize attack. The latter two are the best ways to kill a Thrasher; however, a pack-a-punched shotgun can destroy the spores on their body very quickly. For the Toxic Gas attack, the Gas Mask is important to completely negate this attack, allowing you to keep defending yourself against the Thrasher.