Invaders are the largest and most powerful of The Drudge forces. They look like giant, spider-like creatures with 4 legs and are the only drudge that do not have arms or carry a weapon, other than Storm Scarabs. They possess many means of attacking you ranging from sending out Para-Mites to attack, to firing bullets that resemble Shrieker ammunition at you. The invaders first make their appearance in the Pentagon level of the campaign mode where you will face one as a "boss" to complete the level. After that you see them again in level 7 where you will face them multiple times. Invaders have three main weak points, identifiable by your aiming reticule turning red: two sides of its "face", and a small spot on the Invader's bottom.
A good strategy for fighting against the invaders is to stay far away and attack with long ranged weapons. Close quarters combat will allow the invaders attacks to hit you quickly since you have little time to dodge and a few strike from his attacks will kill you. Use of a scoped weapon is highly recommended.
Another highly effective strategy is the use of radiation grenades. They can deal significant ammounts of damage to Invaders when thrown onto weakspots, and they are also able to automatically kill off Para-Mites as they spawn. On lower difficulty settings, all it takes is two or three well-thrown radiation grenades to take down an Invader very quickly. Frag grenades are decent, but hitting a Para-Mite as it flies by will waste the grenade, and plus that might damage you too.
Unsurprisingly, Invaders boast immense strength and durability. This is further proven in a television broadcast in the campaign mode's Mission 7 (Homeland), where a reporter states that a single Invader was able to completely destroy the military's defence lines.