The sight of the on-rushing army of the infected would have been enough to send anyone running the other direction. But Pete wasn't out there for himself, he was out there for everyone who was depending on him and the other X-Men. Logan's presence helped to reassure him; the crazy rodent zeal was contagious.
Piotr transformed to his armored state and said a prayer in his mind for those behind him about to fight and those before him he was about to harm. When it became clear that the mass was going to reach the wall, Colossus took off in a dead run toward them. When he collided with the lead runners, there was a terrible noise that went unheard in the chaos as he smashed through their bodies and trampled the next wave. Granted, it only had eight or ten feet of immediate range, but he was using them against each other. He began snagging up bigger individuals and slinging them into small groups with most of his strength.
He was now thick in a sea of diseased Darkseekers. They had descended on him and were clawing, biting, kicking and punching every inch they could get access. It felt almost like being under water - his movement was limited and he felt as though he were in slow motion. It wasn't long before he could see little of the rest of the crowd - he had only his armored flesh to thank for the fact that he hadn't been torn limb from limb. He fought to crouch down into a ball and forced all his energy upwards, sending a few dozen of the creatures into the air in all directions. He was only free of burden for a few moments before he was swarmed again and he repeated his trick. In a setting like this, they didn't seem to learn.
He turned back to the wall and realized a section of the wall had been torn through and he tore through the crowd like a bowling ball through cardboard pins. They were funneling into the grounds like rushing water. He had to back up the hand-to-hand fighters that had been waiting inside. He fought his way to their entry point and planted his feet as firmly as he could - attempting to knock back as many of the creatures as he could. They were climbing him in a rush and beginning to scale the torn section of stone wall. At least he'd plugged the leak for the moment.