From The TimeSplitters Wiki

Sentrybot is a character who appears in Timesplitters 2. It appears in Robot Factory, and is one of the reasons Robot Factory on hard is avoided by most fans. The Sentrybot is so powerful, it takes too many shots from the weapons, so most players use the laser security lines or the turret to take them out.

Sentrybot has no head, so no headshots can be inflicted on him. He is a large character with slow speed and is stunned when shot by the Electrotool.