The Seagull Fighter is a bomber aircraft that is used by Mental's forces. Its only attack is to drop several bombs on a specific location. These bombs do quite a lot of a damage, especially if multiple ones hit a target.
The Seagull Fighter is a bomber used by Mental's forces that was being developed on Magnor when Mental defeated that planet. He found it in a secret swamp base, and immediately manufactured it for himself when he saw that it could easily travel at warp speed and had stealth coating.
Like other flying enemies, the RAPTOR Sniper Rifle makes quick work of this enemy. Two scoped shots will destroy it.
Because of how far away it is, other weapons are not suggested. They lack the sufficient range and/or speed to effectively harm the Seagull Fighter.
Usually, Fighters will spawn in multiple groups, even though it may not seem apparent at first (ie one is killed, but another one will take its place a few seconds later). Therefore, it usually isn't worth killing them.
The best strategy against Fighters is to figure out what location they're bombing and avoid it. They usually bomb areas like paths (such as the dirt path leading to the village in the second part of Greendale). They will constantly bomb the same area until killed, so staying away from where it's bombing will make it harmless.
Once their bombing target has been found, Fighters are harmless.