...it depends on what you're referring to here .

Simply having the climber bash as long as possible? That's a standard delaying tactic, of course it could be made more consistent by making the piece of terrain thicker, but most Lemmings players should know about this, similarly to assigning a floater early or late, depending on whether one wants the fall to be fast or slow.

Bombing through the terrain at the bottom of the pit? Yeah, making it a little bit thinner could help?

The double builder turnaround trick? Honestly, to me that is one of the most obscure tricks ever. I like it a lot myself, but it is hard to teach to new players without explicity informing them about it.
In contrast to the three builder wall from "I have a cunning plan", or the double miner turnaround, both of which could be figured out with extensive logical thinking, I doubt anyone less seasoned would consider the option of a builder "bumping his head" on a staircase that's behind him rather than in front of him.

Especially since the three builder-wall is also a thing, and in that case none of the three lemmings turn around. The double builder turnaround requires one lemming to step over a lemming who is already building, and then start building early enough before the first staircase becomes too high.

For my upcoming pack, I made an introduction level with a pre-level text explicity informing the player that this trick is possible - because I considered everything else to be too unfair. Meaning: The level boils down to "do you know this trick is possible or not?", and if you don't, then you don't stand the slightest chance of solving it.

Level 01: Making miners go through with builders - not an easy start, but not too difficult either if you're used to looking for such opportunities (which I am, since I have been using such tricks constantly in the creation of Lemmings World Tour). Also, I doubt this is going to be the very first level of the pack when it's present in its complete form, so nice job here!

Level 02: The basic idea of using a skills "outreach" to other pieces of terrain to have it pay double duty is great. Plus, the terrain makes you consider that option pretty quickly, due to the close proximity of the two groups of lemmings (i.e. the crowd and the single pioneer). The execution however is ridiculously precise - I almost always had at least one lemming escaping to the right. If this level is supposed to be on rank 2 indeed, perhaps lower the save requirement by just a couple of lemmings. I have level in Lemmings World Tour requiring a similar maneuver with two diggers: In both levels, it is possible to save everyone (or in your case: everyone except for the bomber), but success or failure are mainly up to lucky timing, even when the player has figured out the correct solution conception-wise.
Just like with time limits that are too tight, these are the cases where I'd advise for a slightly more lenient save requirement .

Level 03: Another obscure trick, the double builder turnaround. But nice approach using this one in a place where one actually has to build anyway!
The climber fortunately has enough room to start bashing early on, so that he takes longer to finish the tunnel while the floater is building.
The terrain in the pit where the crowd gets trapped could be slightly thinner, though; since the bomber only works in one specific place, I was thinking for the longest time that bombing out of the pit wasn't possible at all. You might want to reconsider whether you want to gaslight the player like that .

Level 04: This was the one I couldn't solve. I tried stacking two bashers on top of each other to create a tunnel that goes through all the terrain at the top. But that's about where my fantasy ended. I'm always curious about challenging levels without any builders, though!

Level 05: Speaking of gaslighting: Several of these fire traps don't work. They only trigger once the lemming reaches the top of the trigger area, meaning the fire traps close to the ceiling are useless because the lemming never actually manages to climb that high.
If that glitch is supposed to be the point of the level, then expect a lot of complaints for inconsistent trap behaviour (lower fire traps killing the lemming while those close to the ceiling don't).
If you simply didn't know about it, then my solution is probably a backroute - which is quite likely indeed, since I managed to save everyone.
Finally, in the New Formats version, those traps at pillars going all the way up to the ceiling aren't required anyway, since the ceiling is deadly and the lemming is going to climb to its doom regardless of whether there's a trap on the pillar or not.
Meaning if the level is actually supposed to work as in my solution, you'd have to put terrain at the top of the level, because otherwise the conversion to New Formats is going to break it.