I found this was much easier to do by just killing everything rather than trying to catch humans in mid-air. It will keep your humans alive longer which is important because if all of your humans die, the game becomes excessively difficult.
Aside from that, you need basic Defender skills, most of which revolve around fast eyes, fast reflexes and how to deal with the different enemies.
Landers should present no trouble for you on their own. Just be aware that they travel faster on each successive level.
Mutants are aggressive but you can outrun them so just try to keep them behind you. To kill them, lead them in one direction, turn around and let them come to you.
Baiters are a pain, mostly because they are faster than you and shoot a lot. Let them chase you and when they get close try and pull up or down and change directions. If you can turn back around and face them from behind you can usually get a few shots off and maybe kill them
Bombers shouldn't be too bad. They tend to move vertically a lot so really the tough part is making sure that you don't have to try to fly through them. If you do, good luck avoiding all the bombs they leave behind.
Pods are pretty harmless on their own. They don't really do much but when you kill them, they turn in to swarmers which are terrible. They are basically like smaller versions of baiters and they come in bunches. Usually it's smart to try to get as many as you can on the screen at once and just bomb them.