There are eight magic spells that you can unlock throughout the game. These spells use Magic Points, represented by the blue bar at the top of the screen. These will recharge over time. If you know the button combinations you can cast almost all of the spells at the outset. I think the easiest spell to cast is Tetra Spirit. It can also kill multiple enemies simultaneously.
To cast it, press straight up and hold it there for a second and then rotate the direction that you're facing until it's pointed straight down. As soon as it's pointed down, press or . This fires off four magic homing balls of energy that will kill weaker enemies in a single strike.
This achievement is very easy to unlock when you first enter the castle as Alucard. After fighting a few Wargs, you'll come to a point where Zombies spawn continuously. Get to the end of this hallway and there's a point where you can jump up on a low step or platform. From this point the Zombies can't reach you. Simply cast Tetra Spirit repeatedly until you kill ten consecutive Zombies and the achievement will unlock.