Tricky 21
You can solve it with bashers and builders, or miners and builders. To hold back the crowd, you can use either diggers, miners or blockers. So the only thing essential is the builders. The fewest I can do is 3.
Tricky 21: 3 Builders
Tricky 22
It can be solved bombers-only. It can also be solved builders-only.
Tricky 22: None
Tricky 23
As far as I'm aware, there's no solution that doesn't use all three destruction skills, the builder, and one climber... >_>
(By the way, I just noticed that this and Tricky 19 have the same skill set!)
EDIT: Or maybe not. It can be done without the miner. Thanks Proxima.
Tricky 23: 1 Climber, 1 Builder, 1 Basher, 1 Digger
Tricky 24
As far as I can tell, at least one blocker is needed for crowd control. You also need two builders to reach the exit.
Tricky 24: 1 Blocker, 2 Builders
Tricky 25
There's a no-floater solution out there. When you get to the far right column, you have a choice of either bashing or mining through it. All solutions use at least two builders, however. So...
Tricky 25: 2 Builders
Tricky 26
If I remember correctly, there's an over-the-top solution which shouldn't need bashers (you'd only need to create holes in the bridge maybe, and a miner or digger can do that). You can do it without builders by bashing right next to the post, then making the next lemming mine then immediately bash when he's just about to turn around.
Tricky 26: None
Tricky 27
I'll let someone else get this one.
Tricky 28
It can be done with only builders, so all other skills are non-essential. It seems the minimum amount of builders you can do it with is 2, so...
Tricky 28: 2 Builders
Tricky 29
It can be beaten with only bashers. It can also be beaten without bashers. Replay attached. Therefore,
Tricky 29: None
Tricky 30
You can do it with blockers, builders and miners. (Block off the crowd, mine and build across the two lower sections)
You can do it with climbers and miners. (This one is pretty obvious)
The only common skill between these two solutions is miners. Attached is a replay that doesn't use miners.
Tricky 30: None