X-Blades

You will not have 10,000 kills by the end of your first playthrough, but kills carry over between all save files. Even by the end of your second playthrough however, most players will not have the full 10k. There are plenty of methods to grind out extra kills by replaying levels, the most popular of which being to replay the stage “Hall of Columns” (third last level). You get between 350-450 kills each time you complete the area, so grinding out the last few thousand can be done there.

A new method which claims to be faster is on the level “Entrance To The Castle.” Make sure you have purchased the spell “Lightning Range” and proceed to either the left or right area where the Ghost spawner can be found (large red orb). Ghosts will continue to spawn and you can cast Lightning Range on them for quick kills without damaging the spawner. Having level 3 health regeneration helps.

If you haven't unlocked this once you've gotten to Hall of Columns, simply go back and keep playing the level. You get at least 400 kills each time so you should easily get to 10,000. All your kills are cumulative from all your save files and can be seen whenever you leave an area in your stats.