You unlock Witchslayer difficulty by beating the game on Hexen Knight, while to unlock Hexen Knight you must complete the game on Squire or harder. All of your equipment and magic gained from previous difficulties will carry over and it is highly recommended that you acquired them all and leveled them up completely (I don't know how you couldn't have by this point, though). Witchslayer, being the hardest difficulty on the game, is really only so because it allows the game's technical pitfalls to shine through more than normal. While enemies will be far more powerful and aggressive, the real problem will come from two main aspects: Boss QTEs and times when you and Gretch--"The Betch"--en are separated.
When fighting bosses, either have their QTE memorized or have the button-presses written in front of you. Here is a link for reference:http://www.xbox360achievements.org/forum/showthread.php?t=29...
The QTEs on Witchslayer are horrible. Hitting a button too late OR too early in some cases will cause failure and will regenerate a few bars of the boss's health. Matters are even worse for when you fight episode 19 and 20's bosses as failing their QTEs will make you START THAT SECTION OF THE BOSS FIGHT OVER. No bueno. Sometimes what I did was rhythmically (not too fast or too slow) hit the next button that I knew was in line along the QTE. This pretty much insured that I would hit the button along the time that it appeared on screen. When you know a "mash __" button segment is coming up, go ahead and start mashing that button before it even appears on screen. This strategy worked like a charm except for Faust's final form of episode 19, where as SOON as the "mash X" prompt appeared on the screen it said I failed it, but maybe you'll have better luck. Broken? Yes.
The most excruciating moments come when Heinrich and Gretchen get separated. Episodes of note are episode 4 & 12-15, the latter being the most miserable point of this whole game. In episode 4 you will only command Heinrich as you look for Gretchen's unconscious ass. I think there were at least two instances where you must fight a group of 3 Wights by yourself all the while being TIMED, though the timer didn't seem to move hardly at all for me. This part isn't too bad but be prepared to rapidly hit A for several minutes as you spend the majority of your time in pieces.
Episodes 12 and 14 will see Heinrich with Minukelsus instead of Gretchen. Minukelsus's freeze bomb (RT+Y) and powerful sword thrust attack (RT+X) are highly useful WHEN they connect, though his other two skills are only partially useful in a handful of situations. Two main places to worry about are when you are in a room where you fight four(!) dire wolves and later in episode 14 when you fight Shadow Gretchen, aka the cheapest boss in existence. Luck will play a huge factor in these fights as they are extremely cheeeaaap. Minukelsus's two moves I mentioned above are critical to winning them with the least amount of headache, though you will probably miss with them more than your fair share of times. It is important to note that when you get Shadow Gretchen down to about half health it is possible that just repeatedly spamming X on her into a corner will keep her from using any of her stupid ass spells or summoning Wights. This method will take a few minutes but is SO worth it if you're able to pull it off.
Episodes 13 and 15 have Gretchen all by herself, which really isn't as bad as you think. Make use of Colchis' Trap and Maar's Lance to quickly bounce any enemies you see. Also, using Gretchen's charged Blast attack (hold B) will usually kill trapped enemies in one hit. This move is key to defeating Shadow Heinrich, who is a much more aggressive but slightly easier shadow person to handle. Simply bust out spells like Mag Mell's Rose, Elizabeth's Fangs, or Fafrotskies Rain on Shadow Heinrich not to damage but to subdue him for a second. As soon as he stops from the spell's effect, run over to him and unleash a charged B attack to deal significant damage to him. The timing window for this is extremely short so don't be too distant from him when you attempt it.
Other than these moments, the rest of the game is comparable to Hexen Knight and may be frustrating, but remains very doable if you've made it to this point. Good luck and let me know how to improve this guide!