Nice
Well, the shotgun, machinegun and armor-piercing seem a bit useless( and the bashing attack), since most of the time I just burn or electrocute enemys to death. The problem with too much waves...well, it's already been mentioned.
I'm not sure if this is a bug, but appearrantly, the barrels can conducts electricity with out being damaged, so if you buy lot's of barrels and maximize the electro spread range, you're basically invincible.(The current just jump back and forth between the barrel and the enemy until the enemy get killed.)
Another thing I'm not sure if it's a bug is, that you would get backfired by the current if it killed an enemy and ended up flying towards you.
Reminds me of Endless Zombie Rampage(1), but the progress is faster.
Over all, I like it. 4/5!
Finally, I got a question... is the menu music really performed by MasterCastle? I didn't find this song in their album "The Phoenix".