The achievement is not cumulative and so all 20 Nosalises must be killed on one playthrough. While there are a few opportunities to kill Nosalises during the segment as Uhlman, such as when recharging the batteries during the rat sequence, the bulk of the kills can be made after the perspective shifts to Khan.
Khan begins with weapons and must fend off an assault by a pack of Nosalises. Eventually Khan will be trapped in a station overrun with Nosalises, so they are quite literally everywhere. More ammo and weapons can be acquired from dead bodies found around the level. Kill 20 Nosalises and the achievement will unlock.