You get Akuma once you collect all 60 CAPCOM stars. If, like me, you are having trouble finding that 60th star to unlock Akuma try this. I had counted the map, all the layers, upper/ground/lower over and over and still could only count 59 stars. Had even started a new game just in case I had quit out early and one had not counted. Not so. The trick for me was it was on the Fireworks stand between West Ridge and North Peak in the farming area. It was covered up on the map by the Bear Icon (signifying the Firework Stand) and the legend icon for the Arcade Machine (character select). If you are still missing a star and none are showing on your map try this location, see if it's your missing star.
Once you get Akuma this achievement is simple. The Raging Demon attack is Akuma's Y+B, and hit's multiple enemies around you. So, charge up your combo, and walk into a crowd of zombies and unleash. I got this in three tries, the first attack killed 40-45 zombies alone. Also, the 2nd costume trial for Akuma has you killing either 200 or 300 zombies with his special. You can easily knock this achievement out then as well. Good luck and happy gaming.