Erron Black
The Outlaw
I never said the easiest or most reliable source.Raiding is only the better experience option if you don't mind spending loads of coins/money on Raid Passes and Lucky Eggs, as well as having the ability to raid multiple times in a row in the first place.
I just said best source. In other words... you get a lot of EXP in a short amount of time with next to no effort other than jumping in and tapping on your device repeatedly until it's defeated.