Grunts with shields: The easiest way to defeat these like buggers, is to use coring charges with a slingshot and just fling away. After a few or couple of direct blasts, its shields should get destroyed, leaving it's weak chassis vulnerable.
Troopers: These are usually the hardest to hit, but if you have the correct firepower and accuracy, you can take them out quickly. If you want the quickest kill, use a level 3 scatter blaster, get close enough, and blast it to metal shards. If your accurate enough, use the level 3 rivet gun and a single clip should blast them out of the sky.
Predators: These can be a real pain if you dont aim for the correct spots. With a rivet gun, barrage cannon(level 3 rocket launcher), or a stationary machinegun, aim for all four arms first. They are easy to disable or destroy, and once you take out all 4 arms, its completely defenseless.
Titans(no shield): with an accurate machinegun, or a level 2 or 3 rocket launcher, these go down easily with shots to the torso or head. If your too weak and desperate for a quick kill, use one full cleaner.
Titans (shielded): If you have a nice supply of EMP grenades, lobe one of these covertly with a slingshot to first disable it, then use a level 3 rocket launcher and just blast away before it wakes up again. For a quick kill, use 2 cleaners, and a couple rockets if its still alive.
Mortars: These can take a pretty good toll on your health if you cant find them quick enough. Once spotted, use a level 2 or 3 rivet gun, and 2 full clips should be enough to take them out of commision.
Setrys: Destroy these without them seeing you can really save you from its deadly barrage of bullets and rockets. Use a level 3 rivet gun, empty 2 full clips, or use a level 3 barrage cannon to finish them swiftly.