I've been experimenting around to find the best place to farm for energie and crystals for the achievements. By far the easiest is on the Terrace - just fly up, kill the one ice lady, and exit out the other side. It's really, really boring though, so I found a better method, at least in terms of fun - and I'm pretty sure speed as well.
I made a short video of it to show exactly what to do:
http://www.youtube.com/watch?v=0L2qHVRvlGsOr if you can't watch: equip Red Boots & Destruction Choker. Go to Clocktower, 1st floor. Go up left stairs, fire 4 spears into the two visible enemies. Fly out, charge shot the one that survives on the left. You'll take a hit, but doing it this way gives a ton of bonus energie for all the shots on screen. Then kill the remaining enemy normally, exit and repeat.
If you ever run out of power, equip Shaman Earrings and Dark Brooch and wait for MP. Every time you get a bomb ready, replace Choker with the Gold Torque, and use bomb instead of the 4 spears. Replace Choker and continue.
Because I have them, the average energie/second numbers I have are:
First 2 enemies in Library: 42.8/s
First enemy in Closet: 54.3/s
Ice enemy in Terrace: 70/s
Clock tower method: 100/s NOT counting down time when you run out of MP.
If you're wondering - 500k energie is more than you'll ever need basically, but 30 crystals won't even get you to max HP/MP. Once you have enough energie, you might be better off finding a quick place to farm small enemies for crystals IF all enemies have an equal chance to drop one. But I don't know if they do.
Now, if anyone knows where and how I can get a 150 hit combo, let me know.