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just shoot the window and the Malthusian will be swept away

Swept Off Their Feet is an Achievement/Trophy in Hydrophobia. It is earned for knocking over 3 Malthusians with water. At the start of Act 2, there are several glass tanks filled with water. As a Malthusian walks past one of the tanks fire an overcharged Sonic Round shot at the glass, which will break the glass and knock over the Malthusian. Repeat twice more and the Achievement/Trophy should unlock.