To you're edit. I actually completed this while running that strike 5 times for a 5 strike no death bounty so it is do-able even with randoms. The solo was what I would have done after if I still had not gotten the bounty. Just get super aggressive once the walker looks like its on its last leg. I always hang out behind the building next to it and shoot it through the tunnel when its attention is on the others. That should get you close enough to grenade/super/ heavy gun it on its final down.