Okay, so once you've gotten to the 10th or 15th floor, you'll find that Dredmor is really, really hard to beat. I've died to him quite a bit in the past few days, and only just now found a way to quickly and easily kill him without exploding into a million pieces first. First of all, you're going to have to choose a good lineup of skills when you start the game. You're going to want to be able to make a few specific items for the strategy I have in mind, as well as some optional skills that will make it a bit easier.

• Alchemy (to make potions)
  
• Tinkering (to make Clockwork Drill Bolts)
  
• Archery (to shoot your bolts better)

• Archaeology (Ancient Krongian Ritual will help you tremendously)
  
• Smithing (to get really good armor quickly)
  
• Clockwork Knight (boost to your clockwork-based attacks)
  
• Perception (helps sight radius and ability to hit target)

There are several items you'll need to carry out this strategy.

• At least 30 health potions
  
• At least 20 invisibility potions
  
• About 15-95 Clockwork Drill Bolts, reserved for the final boss
  
• A really good bow, beefed up by Anvils of Krong as much as possible*
  
• A really good set of armor, beefed up by Anvils of Krong a bit*

*Remember not to use the Anvils of Krong on early items, and to save them for the really really good ones!

It will be easy to come across some of these items, but let's talk about the ingredients so that you can make even more of them.

• Aqua Vitae (Healing and Invisibility Potions) - This can be made by putting any kind of alcohol through the Porta-still.
  
• Rust (Potion of Healing) - This can be made several ways via the Elven Ingot Grinder.
  
• Bituminous Coal (Potion of Invisibility) - A bit trickier, since it can't be created. However, it is a common find on most floors, so it should be rather easy to find.
  
• Spring-Loaded Power Core (C.D. Bolts) - Can be found as a common drop on several floors. Can be created from a Steel Ingot and Brass Mechanism.
  
• Brass Mechanism (C.D. Bolts) - Can be created from a Brass Ingot.
  
• Steel Ingot, Brass Ingot, etc. (C.D. Bolts) - Can be found as common drops, as well as created from metal-based armors via the Disposable Ingot Press.

Note that to make the Clockwork Drill Bolts, you might need to find the recipe to create it. If someone knows whether this is a bookcase recipe or not, please tell me.

• Clockwork Drill Bombs
  
• Holy Hand Grenades (cannot be crafted!!!)

The strategy I'm using works on any gamemode, and takes advantage of Dredmor's weaknesses. First of all, to find Dredmor you'll need to explore the level itself. Be sure to kill all monsters in a room before opening another door. If you open the door and see Lord Dredmor, immediately shut the door and get up to full health, then chug an invisibility potion. If Dredmor is standing directly in front of you (and therefor blocking the door), chug an invisibility potion and try to get away from him. This prevents him from hitting you, but he will still attempt to pursue you. You may have to chug more than one potion before you're far enough away (remember, the effects of invisi-potions don't stack, so don't drink them all at once or you're wasting them!).

Next, lead Dredmor into a relatively open room. Get far enough away from him so he's on the edge of your screen and within range, then equip the Clockwork Drill Bolts and shoot him with them. His health will go down by something like 20-100 for every shot depending on how good your bow is and how much luck you have with RNG. Ideally, you want to have a good enough bow that it inflicts 100 points of damage almost every time.

Shooting will break your invisibility, and Dredmor will walk towards you. Keep shooting until you think he's too close, then walk to a door and close it behind you. If you took damage, heal up and chug another invisibility potion and open the door. After playing another game of "catch the invisible adventurer" with Dredmor, you'll hopefully have gotten him into another position where it's safe to begin shooting him with the drill bolts again. Wash, rinse, and repeat.

Eventually, Lord Dredmor will drop dead. In my game, he had 1700 health, so it took around 25 arrows counting the ones he dodged. In another run, he had 1300 health and it took around 80 arrows counting the ones he dodged. But depending on your RNG luck, he might dodge more or less of those.

Congrats! You just got an achievement for killing Dredmor! Please let me know if this strategy doesn't work for you. Also this was my first guide, so tell me how I did! :D