Weak pokemon
1. get any weak pokemon that you have
2. make sure that you can beat the elite four really good
3. give the weak pokemon exp share
4. battle the elite four
Strong pokemon
1 you'll need a lucky egg (explanation of how to get lucky on the bottom)
2. give your pokemon the lucky egg
3. let it battle in a place where there are strong pokemon ( if pokemon at lv 50 go to a place like the resort area and keep on battling on the grass or batle the trainers using vs seeker)
Lucky egg
1. get a pokemon with compound eyes ( to help you find it easier) and a pokemon with thief
2. make sure the pokemon with compound eyes is in the first slot
3. look for a chansey ( on route 209)
4. once you found the chansey make sure you use thief or catch it
5. finding a lucky egg will be really hard because not all chanseys have it