We saw the Monster get seriously harmed by bullets, a bear trap, a baseball bat with nails, and LITERAL FIRE. There are clear signs that it was wounded by this. Yet shortly after, at the middle school, it's able to easily outdo trained government agents with machine guns. Is this a plot hole, or is there a convincing idea about why this might be?
My working theory is that monsters from the Upside Down get stronger when they come to the brink of death. Perhaps this is a sign of things to come in Season 2?