Fewest repeats of the same skill to beat levels.

[Proxima]

Returning to glitch-free results, Wicked 1 with 2 is obvious, Wicked 10 seems to need 9, and for Wicked 3, both the left and right side routes use all eight builders and I can't find a way to avoid this.

[namida]

I updated the first post, but I see I didn't post the reply. I had a go at Wicked 11, I did it with 4 (using 4 each of bombers, blockers and builders), but I see the skills you can't live without has only 2 builders. What was used of other skills in acheiving that?

Gonna try some more levels now. Might see what I can fill in on L1.

Mayhem 26, we have a starting point of 6. Bombers, surprisingly, not builders (which came in at 5). If you can cut one bomber off this solution without using a builder... (you DO have two blockers to spare, and it saves well above the required number of lemmings).

The rest of them... I am not touching those with a 10 foot pole, except maybe Mayhem 18...

[LemSteven]

I updated the first post, but I see I didn't post the reply. I had a go at Wicked 11, I did it with 4 (using 4 each of bombers, blockers and builders), but I see the skills you can't live without has only 2 builders. What was used of other skills in acheiving that?

Wicked 11 is confirmed with 3 (see spoiler).  I'm pretty sure that the 2-builder solution requires 3 or 4 bombers, so that won't work here.

Use a digger and a basher to handle the beginning and then mine through the tree.  Once through the tree, the miner blocks, and another lemming blocks the left side before the trap.  I tackled the next obstacle with two miners and a bomber, and after that the route was pretty straightforward with three builders and a basher.  A final bomber releases the crowd.

And now for the rest of Wicked:
12: 6
13: 7 (glitch)
14: 1 (glitch)
15: 6 (this appears to require steel glitches; let me know if I'm wrong)
16: 1
17: 4 ***This improves on the WSCYLW result; see that thread for details
18: 5 4 -- I'm thinking 3 may be doable, but I'm struggling with getting the bombers to work in my favor.
19: 5
20: 1 (glitch)

[Proxima]

Very nice find on 17 

I've had a quick pass through those to set preliminary glitch-free records, but I expect some of these can be improved.

Wicked 13: 14
Wicked 14: 2
Wicked 15: 9
Wicked 20: 3

[namida]

Havoc 1 gives a pretty easy 5. I don't think any less would be possible, due to needing builders to cross gaps.

Havoc 3 also gives a very easy 2. Havoc 5 is quite obviously 2. Havoc 6 can be done with 2 using a direct drop.

I can get 5 on Havoc 19, but it's probably improvable to 4.

[Proxima]

Havoc 4: 1 (ccexplore's floor route)
Havoc 8: 4
Havoc 9: 13
Havoc 11: 8 (glitch) from WSCYLW; glitch-free best is 18
Havoc 12: 9
Havoc 14: 4
Havoc 20: 7

[Proxima]

Havoc 2: 9 (EDIT: this is now glitch-free.)

Havoc 6 seems to need 5 miners for glitch-free.

Havoc 7: 6. The fewest-builder solution is to use a climb-bomber on the right side; then you need 6 climbers to save the required 15/20.

Havoc 19: 3, glitch-free.

[namida]

Even direct drop is considered a glitch these days? O_O Shows how long I've been away...

Anyway, that just leaves 13 and 15-18 for Havoc, then there's a few levels that never got a conclusive answer: Fun 15, Taxing 4, Mayhem 14, Mayhem 18, and Wild 12.

I've been having a go at Mayhem 18, and I've come up with a decent plan in general, it's just down to pulling it off well now.

[Minim]

On Mayhem 18, I don't know how this could be improved in any way. I looked at the 8-builder solution made years ago but I think 8 is as far as it goes. I don't think this will work in 7 considering the number of bombers and blockers you have to use to break the steel.

As for Fun 15, with thanks to whoever made the test replay, I have expanded on that perspective and confirmed that it's possible with 5 of each skill! Here's the replay.

[namida]

On Mayhem 18, I don't know how this could be improved in any way. I looked at the 8-builder solution made years ago but I think 8 is as far as it goes. I don't think this will work in 7 considering the number of bombers and blockers you have to use to break the steel.

Well, considering that my best effort so far used over 10, you're doing well. =P

[Proxima]

Or rather, Clam is doing well, since it's his replay 

Here's what I have for the rest of Havoc:

2: 9 glitch-free
13: 5 glitch-free (WSCYLW says 3, I can't match this yet)
15: 5 (glitch). No idea what the best glitch-free is yet; I feel a shortcut route ought to be possible, but with the skills and save requirement so tight, I'm struggling to get anything to work.
16: 3
17: 2 (glitch) or 4
18: 8

[ccexplore]

Here's Havoc 13 with 3 builders (and 2 blockers), not sure how you want to classify it. Actually having done the level, I'm now thinking WSCYLW for that level can be improved to 2 builders via sliding (similar to the Crazy 8 2-builder solution).  Must try that some time.  (Though not sure whether it is possible to keep it down to only 2 blockers on such a solution, so might not help with this challenge anyway.)

[Proxima]

Awesome solution  It seems to be clearly glitch-free; I'm not sure which part you were wondering about?

Here's Havoc 15 glitch-free with 10, which I'm sure is improvable, but at least it's less than the normal solution to the level!

EDIT: Got 9. Not sure whether 8 is possible.

[ccexplore]

18: 5 4 -- I'm thinking 3 may be doable, but I'm struggling with getting the bombers to work in my favor.

It took a little trial and error and some precision but here, down to 3 for Wicked 18, no glitches (the steel destruction is clearly incidental).

[ccexplore]

Actually having done the level, I'm now thinking WSCYLW for that level can be improved to 2 builders via sliding (similar to the Crazy 8 2-builder solution).  Must try that some time.  (Though not sure whether it is possible to keep it down to only 2 blockers on such a solution, so might not help with this challenge anyway.)

It worked out nicely thanks to the terrain effectively saving me a blocker, so not only did I improve on WSCYLW and least-number-of-skills, the sliding solution also works for this challenge, improving Havoc 13 to "2 (glitch)".