First, spawn a pig or look for one when it's raining. You may want to trap it on a high platform. When u trap the pig, wait for lightning to strike (if u are impatient use a mod) The zombie pigman will spawn. Same with the charged creeper, shock it with lightning.



Note: I did this on my friends server and I Found it. May need to be on a multiplayer server: lightning doesn't come in single player