I *HATE* that damn guy! Always when youīre in a hurry or fighting someone it sneaks up from behind and grabs the weapon your using at the moment leaving you in big trouble. Itīs so fast youīve problems to follow it even with the Afterburner. Fantastic AI though!
How to kill the Thiefbot:
Actually there are two (!) ways to do it whereas it mainly depends on your patience and skill which one you prefer; however, itīs always better to kill it right from the beginning. Concentrate on him first so that the level exploration is less hectic. Sometimes youīre fighting bots and suddenly your beloved Gauss Cannon is snapped cause you didnīt hear him creeping up behind cause all of that battle noise. Certainly, if you like more action just try to watch your back and kill him while exploring the mines. Good luck.
After you encountered it place yourself somewhere where you canīt be attacked from behind; preferebly at the end of a long hallway and wait...
1.) The Smart Mine method
Place some Smart Mines in front of you (not too close to yourself!) and turn your back against them. Soon youīll hear him appear and heīll definitely run into the mines and take a lot of damage. It depends on how good you placed them and youīll need only 2 tries to kill him.
2.) The Flash Missile/Gauss Cannon combination
Just wait until itīs very close and then launch a Flash Missile! It gets stunned for a few seconds in which you should attack it with e.g. the Plasma or the Gauss Cannon. Meanwhile fire another Flash Missile so that it canīt recover again. If you didnīt make it and the Thiefbot flees do not follow behind! Just stay where you are and wait until it comes back (always in less than 30 sec.). When it comes up to you again wait until itīs very close and release another Flash Missile. It doesnīt matter if you got affected by the explosion cause when you destroyed it itīll leave behind up to 4 bluespheres and 3 energy But watch out: do not stay too close to it when it explodes!
One thing I experienced was that when I waited behind a closed door and it got opened by the Thiefbot it somehow got "surprised" by myself and didnīt flee as fast as usual as I opened fire with the Gauss Cannon.