Enemies fire their weapons faster and more frequently.
After you save both groups of Marines, continue until you reach the first outdoor area with dirt and vegetation. Take a right as you enter this room and run up to the wall past the Gravity Lift that leads up to the sniper point. Get on top of the wall by going up the dirt mound on your left. Follow the wall and an invisible Gunner Grunt will spawn wielding a Fuel Rod Cannon. Kill him and reach the floating Skull called "Angry."
Enemies' weapons have a higher rate of fire and will use them more. Thus enemies will tend to use their weapons, even being on close range. This voids any tactic with getting enemy hunters or elites to go into a melee attack to avoid them shooting.