While they fought the light infantry in front of them, Max and Nico fired over their shoulders at the infantry coming down the tunnel the moment that they came into view.
They had the same durable shields leading the charge, so it was only slowing them down a little, but the impact of the Mass Driver rounds halted the advance even when it didn't pierce the shield's plating.
That bought them a precious few seconds to deal with the enemies that they were already engaged in, and just before the reinforcements reached the bottom of the ramp, Khan and her partner managed to knock their last remaining opponents back to the tunnel and fill it with Fusion Flames again.
The tunnel itself gave an ominous groan as the flames set off the explosives that the troops were carrying, and the resultant shockwave nearly knocked Max from his feet and threw his opponents to the ground, where they were easy targets for his claws.