To get AP and Stigma(which we can use to get Penguins to level heroes or sell them for Gold) we were farming in Unrecorded History but now after the origin update the best way to farm AP and Stigma is proven to be the last stage in (Season 5 Chapter 10S Connection Episode: Chase 2) it cost 20 energy and takes a bit of time but it's worth it to farm in it, if you can't farm in Season 5 than just farm the previous seasons, but the higher season you can farm the better.