For those of us experiencing a rediculously fast rush of impossibly overwhelming forces in mission five, I have the soltion:
DOWNCLOCK YOUR CPU to 100MHz
Your PPC may have a built-in utility, but if not, just download a free demo of XCPUscalar from Handango.com. This game was obviously intended to run on much slower processor's because our Xscale 400's are actually increasing the game speed dramatically, yet the countdown timer remains the same as before. I know this only because I just revisted this game on my old 200MHz StrongARM.
Slowing your processor to 100MHz will dramatically kill the frame rate as well as game speed (i.e. unit movement and battles take MUCH longer to finish), but it will NOT affect the timer, so you effectively create a "time warp" for yourself, enabling you to hold off the attacks for the 5-minute duration, which finally makes this mission possible.