Really? The Achievement FAQ makes it sound like I could steal more than one.
Well, blast. I guess there goes that idea (it said you could max out the "speed" *I assume the agility* stat from stealing these. So I figured what the hell... I already dumped 80 hours into this, what more is 12?) Guess I'll defeat him and then start to rage at the Mechat Shooting stage 3 achievement. :/
Actually the 'Steal & Run' method works on Gold Mecha Robo too, but don't forget to leave and reenter the area each time (can easily done by warping from Ancient Factory to Ancient Factory, cause it counts as new entrance to the area)