Fighting various forms of aliens happens frequently in Prey and can be challenging even on the normal difficulty setting. Aliens are divided into several groups. Mimics are the most frequently encountered one. First and foremost, remember that these monsters are great at launching surprise attacks since they take shape of the environmental objects. Therefore, you should be careful while exploring unknown areas and always be ready for combat.
The "regular" Wrench does well against singular Mimics in most cases. Remember not to swing this weapon too frequently because otherwise you might exhaust the Stamina bar (described in detail later). Should you run into several Mimics, it's best to start the fight using the GLOO Cannon. Gluing these monsters will temporarily slow them down or hold them in place entirely. Then, you can start attacking them with the Wrench or the Silenced Pistol (don't waste ammo for better weapons on Mimics).
Phantoms are another form of aliens. Even singular specimen are very dangerous. These monsters not only are very quick but can also deal gigantic damage. Moreover, they also come in elite variations - for example, Thermal Phantoms deal additional fire damage.
Using the GLOO Cannon first is also a good idea against Phantoms. After you've slowed them down, quickly switch to the Shotgun and open fire on them. Try to hurt them as much as possible before they come back to full strength.
Sometimes you might run into several Phantoms guarding a single area. In such cases, don't try to engage them all at once but rather try to draw them away to neighboring locations one by one and fight them one at a time.