Clearing the mountain wasn't difficult considering the power of the mechs. Most of them were able to take out two or three of the monsters before sustaining quite a bit of damage since they weren't that proficient in dodging their attacks.
Since the pilots were low level, it's hard to guess where the attack might come from. There were a few who were able to dodge but that's mostly thanks to guessing the direction from the way the monsters moved their bodies. But this method was like a coin flip.
Within the pilots, there was a small portion that could just keep attacking without letting the monster touch them and Glen was included in this.
Overall, Shiro couldn't help but pity the monsters a little. They should be a symbol of fear but instead, an army of mechs charged into their home and killed everyone.