It doesn't seem humanly possible without using a Time Extension. Even then, I had to reset a lot before I finally lucked out.
The strategy is pretty much the same. Line up chains in the left and right columns. If you have extra turns before the boss appears, pincer full columns of junk (initiate with the weapon units whenever possible) to hopefully spawn in more of the colors that you need. Maybe I'm missing something, but it still feels like total RNG luck.
Hmmm... I didn't even think about using time extension. Thanks for the heads up. I agree it's very RNG based though. I've had some attempts where I almost won because I got really good lineups from the start, and some which were a dud from the start because the colors were too spread out. The biggest problem I have is not lining up the colors, but lining up the colors with the characters in the right placement so I can actually kill the row in a single pincer. It's easy in Puzzle Quest 1 but a total bitch in Puzzle Quest 2.