Pretty good, but nothing new
You did a fairly good job on this game; the gameplay is solid and I did not encounter any bugs or glitches (not even the locked movement bug that can occur in flash games), and I played up through wave 23.
The graphics are decent, and the weapon selection is nice. What truly detracts from this game however, is the complete lack of originality. There are countless other games of this type-- viz., isometric shooters in which the player has to fend off increasingly numerous waves of enemies, and in which the best strategy is simply to lead the enemy in circles whilst pouring ordnance into the crowds that gather.
There isn't really anything that makes your game stand out from all the other similar games, besides the fluid, lag-free gameplay and well-balanced weapons (and these things should be standard, not exceptional to all such games).
The only other negative point about your game was the speed of the enemies. Whilst the player can deal with this by simply running backward in a more or less circular course around the level, it doesn't make sense for the enemies to all be that fast, given that you called your game 'Epic Zombie Killer.' Your game doesn't capture the feeling of fending off hordes of zombies (as portrayed in countless other games and many films). Zombies are disturbing not because of their speed, but because of their resistance to damage, persistence, and numbers.
In short, it would make more sense to have slower, but tougher enemies that appear in larger hordes, perhaps with an occasional fast one mixed in. As you currently have it, the enemies aren't exactly what comes to mind for most people when they think of the word 'zombie.' Overall a good game, but rather lacking in replay value.