For this you need to have to purchase the following from the Powers Menu and be at least two sub-stations into The Warren (Chapter 22 - Playing Hero).
Lightning Bolt:
Thug (400 XP)
Outlaw (2,900 XP)
Infamous (12,000 XP)
Electromagnetic Shockwave:
Thug (800 XP)
Outlaw (1,800 XP)
Infamous (2,800 XP)
Arc Lightning:
Thug (5 Evil Missions)
Outlaw (10 Evil Missions)
Infamous (15 Evil Missions)
Shock Grenade:
Thug (800 XP)
Outlaw (1,900 XP)
Infamous (5,500 XP)
Megawatt Hammer:
Thug (800 XP)
Outlaw (2,500 XP)
Infamous (5,300 XP)
That makes it an accumulative 37,500 XP and 15 Side Missions.
Additionally if you played on Hard, had beaten the game, finished all of the side missions, and collected all of the blast shards (Like I did), and don't want to run around in a desperate attempt to find XP for kills because you just didn't do enough in the missions or don't want to start another new game on a lesser difficulty.
Then I've found a bit of an XP boosting glitch;
Pre-note: I recommend being evil so the citizens hate Cole and wish to harm him, because for some reason it seems nearly impossible to get this right on a grounded enemy. The game needs to think someone is attacking Cole, so you're able to get a "Head Shock" on a pedestrian.
First: To be safe, let them throw a rock at him, then punch or shock them. If you need them back on their feet for any reason Cole can always Heal them (
then
).
Second:
This is the part that can be hard to achieve. Cole will then use Bio Leach (
then
) to run over to them and he
must not make contact with them. You'll see Cole jogging in place trying to get into the right position, but fails. The scared look like you usually would if you were trying to leach their life, but he hasn't made contact so this is perfect!
Third: Take a few steps back (as to not hurt Cole) and use you're Lightning Bolt (
then
) to no end for 1 XP per hit. It racks up pretty quickly so go until you're done enough damage. You can use you're Arc Lightning as well, but it drains energy.
Post-note: At times the pedestrian may vanish if Cole strays too far, or sometimes the game magically thinks you're dead by going to a black screen for a few moments then you will have just died (for no reason at all).
If you're not able to get it to work, run around The Warren and look for the Conduits that spawn the trash bots. Just kill them as they're a one shot kill (3 XP per bot) and they will spawn endlessly as long as you don't accidentally kill the Conduit.
If you like this solution let me know, and if you have any concerns I'd appreciate a comment before you dislike. Happy Completing!