Besides you can get 6 of these units as attackers, that should be enough to deal wioth the bastions, and under 650pts.What you need are units to crack the bunkers, before overwhelming the men inside. Small units of stormtroopers are very good for this. Units of 5 with two meltas, using the 'Airborn Assault' mission. This will allow them to deepstrike on target, and then hit the bastions with their meltaguns. The odds of them cracking it open are pretty good.