The 100 hit combo may be a bit tricky. Anytime you are hit by an enemy your combo resets to zero, and if you take too long between hits your combo will decrease. This video shows a method that with a little bit of perfecting, is easily the best and more efficient way to grab the two combo achievements.
The key to this guide is the boss and his weakness or rather lack there of until all of the souls have been purified. With two swords (two broken swords work as well), and leaving one soul (preferably blue) still alive you can follow the guide and until you've hit your 100th combo slice.