I simply didn't see it in the list since it wasn't in the expected position!  My fault.

Good luck everybody!

Difficulty comes mostly from 2 factors in my opinion.

1.) The unexpected usage of tricks.
2.) Entropy. Or a bit more precise: Having many meaningful possible things to do with the limited resources you have.

To further explain point 2:
Let's say you have 1 builder and 1 water pit in the only path to the exit with no other workarounds. The builder is 100% locked in that position and is therefore no mystery. 
Having multiple possible routes where that 1 builder can really help you out on the other hand makes the choice unclear. Bonus points if the meaningful usage is even hard to spot in the first place (hidden in plain sight).
If this holds true for not only one but multiple skills (+ adding some unortodox trick usages) the level can quickly become a very hard nut to crack (if there are no glaring backroutes of course).

One approach is not to worry:

• Make several levels of medium difficulty.
  
• Have others playtest your levels.
  
• What levels did they find much harder than you anticipated? Those are hard levels.
  

My take on Proxima's idea:

• Place some terrain and give plenty of skills.
  
• Solve your level several times. Optimize the solution for skills.
  
• Trim the skillset to match your optimal solution.
  

Or:

• Play lots of levels by others.
  
• Build a level that combines two ideas from existing levels.
  

geoo's favorite:

• Play lots of levels by others.
  
• Submit your replays to the author.
  
• The author will judge some of your solutions backroutes. Sometimes, he'll also fix his levels to prevent your backroutes.
  
• Which backroute did you like the most? Were you surprised that he judged a particularly cool solution a backroute?
  
• Build a new level that has your cool backroute as the intended solution.
  
• I recommend to make the new level look sufficiently different from the original level. After all, the idea is now different, and it's nice to give different ideas different looks.
  

Draw levels on paper, mainly to brainstorm, but you can also plan details.

It's not necessary to start with an idea. You can make your level look like art first, then draw inspiration from that.

Not only the level can look like art, even the solution can look like art. E.g., when you solve a level by Pieuw, ask yourself: Where can we place the basher/miner so that it continues the longest? Now, if your level has only one miner, has no other destructive skills, and has lots of obstactles to mine away, it'll be easy to spot the miner placement. Can you disguise* the beauty? Will the player have an a-ha effect when he finally finds the idea?

*) By disguise, I don't mean invisible things, e.g., don't hide exits/traps fully inside terrain. Everything should be clearly visible and its behavior should be obvious. Still, you can make the player wonder which parts are important and which are merely decoration. As Icho describes: Hiding things in plain sight.

You can make levels harder by adding extra problems to a medium-difficulty level, but you'll have to test for backroutes from the extra skills.

-- Simon

I am going to be pushing the difficulty meter, so to speak, in my next rank. I'd like to make it (as well as some of my existing levels) have more of a sense of difficulty.

Level design is no big deal for me at all, but trying to come up with a good, tough strategy or two for a level is where I am struggling. Any suggestions and ideas for level difficulty would be greatly appreciated!

Congrats on releasing your first pack!

Since it's a relatively short and easy one I took time to play it all. Not sure if it's useful but you'll find my replays attached. Most levels seemed open-ended and I often tried to save as many lemmings as possible.
It's a fine pack even if it gets a bit repetitive in the end because the difficulty never really takes off. Otherwise, it was fun to play! Keep it up