Use the following trick if you are finding it difficult to unlock all of your powers from only missions. After you have built up your character enough to be somewhat dominant (see recommended skill set below), go into an area where you have not restored electricity to yet. Once there, you can grab the attention of multiple enemies by zapping them. Get the attention of as many enemies as possible and they will all chase after you. Lure them into an alleyway that is narrow, then annihilate them. Use any powers that you have unlocked up to this point to ensure your survival. You may find other battle tactics to be more useful which you can develop and use for yourself, but the battle tactic that listed here is recommended. Note: Once you lure a large group of enemies into an alleyway, throw lightning grenades/bombs at them and use lightning strikes to fry them. Throw the grenades/bombs at enemies that are bunched up together in order to get multiple kills at the same time. The more enemies that you kill at the same instant, the more experience you will get. Once everyone is dead, if there are any wounded enemies that are still alive but are struggling on the ground, use the life leech skill (R1 + Square) to suck the neural electricity out of them. This will restore all of your electricity/energy power so that you can repeat the same tactic. Be careful; when using this strategy watch out for turrets mounted on trucks. They are usually located at intersections and up on the railways. A good way of destroying them is to zap the front of the truck, where the engine is located. The entire truck will explode and it will usually take out the turret as well. It is recommended for you to be pursuing an evil karma state, as the Life Leech will give you bad karma. It is recommended that you build up these skills as much as you can before trying this tactic.
Lightning Strike Lightning Push to deflect rockets or push enemies away from being too close (not required but recommended) Lightning Grenades/Bombs Life Leech Sniper (can be useful if you would rather just snipe the enemy behind the turret) Note: Try upgrading all of the skills that reduce damage taken by Cole. Also upgrade the increased electricity absorption rate skill.
Find a Dustmen Conduit in The Warren. They frequently spawn three small scorpion-like creatures that are easily killed. Find a safe location (for example, the top of a roof) where the Conduit cannot shoot you with his rocket launcher and the spawned creatures cannot reach you. Keep attacking the spawned creatures for easy experience.