So, I decided to take a look at L3D in this challenge. While it probably won't be as interesting as the L1-type games, the large number of backroutes might at least make it somewhat interesting.
Here's some baseline suggestions for (most of) Fun.
Fun 1 "Take A Dive" - 1 turner
Fun 2 "That's Right" - 2 diggers (don't worry about crowd control, just use the diggers, and you'll save 4 more than required)
Fun 3 "The Bean Machine" - 3 builders
Fun 4 "It's A Run Around" - 1 turner, 2 diggers
Fun 5 "Bounce Bounce" - 7 turners
Fun 6 "It's A Blast" - 4 bombers
Fun 7 "Candyland Climber" - 1 bomber, 1 basher, 2 climbers (I did quickly try a go-under-the-cane route, but I can only save 3 that way, and at any rate still use all of these skills)
A slight variation on the lose-2 solution completes the level with 1 blocker, 2 turners and 2 builders. The "normal" solution can be done without the blocker and with only 1 builder (at the cost of also needing a basher and 2 additional turners).
The lose-5 solution uses only 5 turners. It is possible to avoid the builder by using the same off-the-path route near the end as the lose-5 solution, using turners to reach that point.
For the record, I did look into whether any improvements could be had by using the invisible blocks near the well, but didn't find anything useful.
Fun 10 "Castle Lemmalot" - 2 builders (This is simply the save-all solution)
The normal approach uses 5 blockers and 4 turners. This leaves 6 remaining turners, which can be used to replace three of the blockers with double-turners.
Fun 12 "Alpine Assault Course" - 1 turner, 5 bombers
Fun 13 "Tower Of Stone" - 13 turners, 2 builders, 20 floaters
Fun 14 "Slippery Maze" - 6 turners (using the invisible route)
Fun 15 "The M-A-Z-E" - 4 turners, 1 basher (I can find three different routes with these skills - one of which makes a
huge improvement in terms of
time taken - but none without them)
Fun 16 "Down The Middle We Go" - 4 turners, 1 builder, 1 digger
Fun 17 "Hole In Ten" - 1 blocker, 2 bashers, 1 floater
Fun 18 "Corkscrew Digger", Fun 19 "Attack Of The 50ft Lemmings" -
(I'll leave these ones to the people who've found existing really-impressive solutions to these two levels, I suck at them)
Fun 20 "Alilemms" - 4 turners, 1 builder