Attack button to punch it with your fists.

Watch for patterns in the hard-to-kill enemies. Some
robots curl into invulnerable balls to fire missiles,
and some stand behind shields. By timing your shots or
moving behind the robots, you can kill them more easily.

If you're not sure where to go, look for the flashing
arrows. If you can't find an exit and there aren't any
flashing arrows, you're probably supposed to climb up.