link again so you can see the Why this is important
http://pokemongo.gamepress.gg/bubblestrat
but basically you want to cathc a CP 10 Diglet for sure, AND then you will need 1 or 2 Poliwag, Crabby or horsea that have bubble attack.
the bubble attack is the important thing.
you then power the poke with bubble up so it is CP 20.
then you put the bubble pokemon in a gym.
fight it with the CP 10 diglet.
Diglet beats the bubble user before the bubble user gets an attack in due to slow animation time.
get 500prestiege and 100XP. rince and repeat.
if you get 2 bubble users into the gym that is 200XP in about 30 seconds.
now you get a friend on a different team to join you, you get the gym to level 10 he clears it out.
you do it again.
this when used with a lucky egg will get you about 20 000 XP if your friend is quick at taking down the gym and you waste no time.