Which doesn't work on higher level missions as lvl 2 and above mission rats easily hit you at range (often they have more range than you do). What I do is have a good shield booster and just try to kill them as fast as possible.
This is not the best way.
As for defenders, never used them myself but apparently there's a bit of a trick to them, namely timing their launch to when you have incoming missles. The more warning you have the better.