This includes the grey turret, the purple "slicer" Droid and the purple crablet-like robot. I am unsure if the plant turret counts. All can be deadly. They appear in the metallic levels. The second challenge, the signal, is a good place to encounter many robots.
All robot enemies are healed with shock damage, and resist acid and fire damage. Blunt damage seems to work best.
The turret shoots >20 fire damage with each shot. Engage from a diagonal to avoid shots.
The slicer deals ~25 damage with its sword arms. It has excellent vision and will pursue you with vigor. It can take a beating, too.
The robot crab is weaker than the slicer, except that it can deal electric damage and physical damage with each attack.