Select normal difficulty, the Gates of Orox level and wizard as your character (make sure you have a spell book that allows you to cast the ice beam spell). Right at the beginning of the level are a set of stairs with two orc spawners on top of them. Make the spawners appear, then back down the stairs to the location shown in the picture below. Start casting ice beam as shown in the picture, aiming it to your left. Now just leave the fire key pressed down.



If aimed right, it should allow you to keep killing any orcs that come at you (even the big ones). I had my character shoot the ice beam for an hour without any orcs making it through, though in my earlier attemps in a slightly different position and firing angle a single orc sometimes managed to close in on me from the left side. If this happens, aim your ice beam slightly more to the left. Even then it might be possible that sometimes an orc makes it through, so it might be a good idea to check back every now and then that your character is still alive. If you've been killed, respawn back in and cast a ring of mirrors to quickly clear your surroundings, then just return to firing the ice beam.

If you do this trick right you should be able to get some 2000 kills per hour (very rough estimate), so even with this trick the achievement takes some dedication and time to complete.

Select hard difficulty, The Demon Pen level, and wizard as your character (again make sure you have the ice beam spell available). Choosing hard for this achievement is a good idea as it makes more enemies spawn, and they should never be able to get close enough to attack you.

At the start of the level, clear the demons to your left. Then check if there is a one of those winged fireball shooting demons behind the bottom bridge. If there is one, shoot it with your fireball (the mortar one) before lowering the bridge. Take care of the initial massive wave of demons that come at you after lowering the bridge (I would recommend the Void school spell book for dealing with them quickly).

Once you've cleared the initial wave, go to the location shown in the picture below and start casting the ice beam. It doesn't matter if the demons come from above or below the ice beam, they should always get hit by it since there are obstacles in their path that forces them to walk through the beam. As with the Orc Slayer achivement, again just leave your fire key pressed down and let the kills rack up.



One thing I noticed is that after an hour of killing demons the game started to slow down, eventually slowing down to a crawl and the camera completely bugging out. It looked as if the piles of ice left after killing the demons on the bridge somehow never disappear, and if too many of them remain on the screen the game starts to slow down. You might want to reset the level every hour or so (possibly even earlier depending on your PC) as it might crash your game. Keep checking if and when the game starts to slow down, then just reset the level and start over (setting up the whole thing again only takes about a minute or so anyway so it shouldn't be a big deal).