Try to get a Pokemon with the ability Cloud Nine or anything else that eliminates the weather. Also go at night. Try to make it fall asleep, but whatever you do do not use a status problem like burn, or poison or anything that will hurt it, just use sleep and parlyzing effects, also I recommend using a Heavy Ball which you can get by giving Kurt the Pokeball Maker a Black Apricorn or also use Timers Balls which you can only get from the Goldenrod Department Store up on the top floor with the lottery.
And of course get it down to red HP.
Hope this helps!