Continuing with Prima...

Taxing 1 - Can be solved with just builders plus a single floater. Minimum builders I can manage it with is 11, and this solution does not need a floater. It's not a very tricky solution, but it does take advantage of an oversight in the level's design that you'd probably notice anyway while attempting this. So: 11 Builders.

Taxing 2 - There's at least one spot where you can dig to solve it with just a single digger (and a release rate change). It can also be solved fairly trivially with just the builders. So: None.

Taxing 3 - Ugh, I hate this level even without challenges. So, I can manage 2 Builders, 2 Bashers, 2 Diggers. By exploiting the exact position of the trigger areas, it's also possible to do with 1 Floater and all of the destructive skills bar 2 diggers (eliminating the builders). The first solution, it's possible to substitute a miner for a digger with very good timing, but I can't reduce the bashers. So it seems: 2 Bashers, 1 Digger.

Taxing 4 - Fairly obvious solution with 2 Blockers, 2 Builders and a Miner. (If you need a hint, cram the blockers as close together as possible and you should be able to work the rest out). A digger and a basher can substitute the miner, but due to the thin terrain and limited builders, I'm willing to say the blockers are the only way to contain the crowd, and 2 builders are absolutely essential to the solution, so: 2 Blockers, 2 Builders.

Mayhem 1: The obvious solution here is a Builder and a Digger. A not-so-obvious one (direct drop xD) can be done with a Digger and a Floater. The digger is very clearly not avoidable. So: 1 Digger.

Mayhem 2: Not going to try this one right now, it's the kind that's very annoying for this challenge. I'll probably come back to it later.

Mayhem 3: A direct-drop variant of the obvious solution can be done with 1 Floater, 1 Bomber, 2 Blockers, 4 Builders. By stretching the builders as far as you can from the edge and placing two more after top-edge-hits, you can do this same solution without a floater. By using every spare builder you have as delays, you can do this same solution with only one blocker. I don't see any way to avoid the other blocker or at least 4 builders, so: 1 Blocker, 4 Builders.

Mayhem 4: The only skill that isn't possible to rule out very easily is builders. As for the minimum of them, it's fairly easy to tell it won't be any lower than 7. While not overly hard, it isn't overly easy to do it either - but 7 is indeed doable.

so: 7 Builders.

WORTH NOTING: The Mayhem 3 1-blocker solution also saves 97% (78/80) lemmings, breaking the current record listed

here. I've posted a replay in that topic.