When you get to the chapter "Ravenholm" you will approach a house and find lots of saw blades. Use your Gravity Gun to pick them up and throw them at enemies. If lucky, you can mow down a whole group of them.

When you find large groups of barnacles they can be an absolute pain! You can't get past them easily, heres one way to do it:

1.Get and explosive (flammable) barrel and roll it into a barnacle tongue.

2.Shoot the barrel with a gun (preferably pistol,NOT CROWBAR) until it is on fire then wait for the blast.

3. Laugh about the hapless barnacle that ate an explosive barrel and killed everything around it.