This is similar to my trophy solution for Agile... Go on a killing spree at night time, keep an eye out for large groups of zombies and have at 'em! Your power points will double meaning you'll level up twice as fast so this is a good way of achieving this.
You'll probably have already reached level 10 by the time you can unlock these abilities but in addition, I also stumbled across a fantastic way of leveling up your power skill tree VERY fast!
Once you reach Old Town you'll find a couple of what seem like town squares, and you'll notice they're over-run with zombies. I found a great way to level up Power skills was to first unlock the 'Kill Frenzy' ability which give you a bonus and stacks your points for killing consecutive zombies, (so the more you kill the higher the bonus.) Also unlock 'camouflage', 'camouflage attack' & 'takedown'.
Now find a zombie nearby to the town square, kill it, use camouflage and then enter the square. Work your way around the square by walking up behind zombies, using takedown to kill them instantly and then move onto the next one. You can kill a LOT of zombies in a very short space of time by doing this and because of 'kill frenzy' your points will start stacking up really quickly. By doing this at night... they'll stack up even faster! You'll be able to do this time and time again without being detected. But remember, your camouflage won't last forever so every 10-15 zombies you'll have to re-use it with one you've just killed.
You may also find that there's one of those big Armour plated tank zombie bastards in the square so it may be worth your time killing him first. Be sure to pick up the loot that the zombies drop as well!