The Dawn of war 2 wiki has an excellent solution to this, so thought I'd share it here. All credit goes to them ( http://dow.wikia.com/wiki/Dawn_of_War_II/Rush_%27em
Just start a new campaign on recruit. Blast through the enemies as fast as possible, and use grenades frequently on larger groups. When you get close to the tunnel, throw a grenade at the tower and blow up the explosives near the tunnel entrance.