Make a 4x4x2 hole with lava in it and place signs on top of the lava.
Now spawn a spider on the other side of the hole, and use a bow to make it aggressive.
The spider will try to walk "across" and fall into the lava even though it could walk around it.
This also works with zombies, (and other aggressive mobs I guess) and you can use fence gates instead of signs, but you would need to remove the lava.