it's not that simple (but what am i to say, i caught Celebi with ease)
if u don catch t fast, it'll run away and u gotta try again, it almost did with me...
Celebi will make 5 vines appear in the field, 1 in every corner, and 1 in the middle, and if ur styler touches it it'll become damaged. Pokemon assit aren't necesary, but make sure to recharge ur plusle/minun's energy cage so u can stop it, but it's luck, cause if it lands near a vine in the corner, well... hope that helps the maker of this thread, and to it's readers as well (i have don the mission, i kno

)