it depends. if it is a gym leader and you know what you are up against, use strong Pokemon that they can not hit hard on. other than that just use your best Pokemon and it should turn out well since all the trainers can be easily beat.
First, you get a Gastly and teach it Mean Look and Hypnosis. Then search for Mesprit. You pick mean look as the first move and then switch to a high-level Pokemon that knows False Swipe. Keep using...
i find it best when u click the mouse on top of it (don't let go) and make a whole bunch of zig zaggy lines and whirls and circles and at the last second let go. one time i beat azula with one turn...
If you got Piplup as your starter training it until its level 90 or more. If you chose Chimchar make it level 95 or more. If you chose Turtwig make it level 100.