Sour 5 / Sweet 2 (major design change, Sour 5 solution change)
Sweet 2 is still very accessible due to the 50% save required. But it prepares well for Sour 5.

Sour 5 was another story, quite hard now ! I used the "turn a miner with a blocker" trick to save miners needed on the right part of the level. Though the hardest part at the end was to turn that miner so that the crowd does not splat. By some kind of miracle and after many tries, I did it and unwillingly made a precise settings which uses "you fall between diagonal connecting pixels". With this setting, not only the crowd does not splat, but they all fall down in the good direction ! So I even saved a blocker here. Not sure this is intended ?

Sour 6 (design tweaks, solution overhaul). Now I find the level actually easier than before, but this may be because i did not find intended last time and struggled a lot with the timer. I think it is perfectly fine now.
The hardest part for me was to figure out how to save some time at the end : if you set the last delaying basher not high enough, the crowd will make a long way before turning back and you run out of time ! Setting it properly leaves plenty time and I could even save 100%

ScottyLems 1 - Crying Over Spilt Lemmings Nice level to start off the pack, with the only hard part being to get the crowd down safely.

ScottyLems 2 - Entanglement Harder than the previous level due to the many steel blocks scattered around in the wall/terrain. However, you are given more skills than you need, so that compensates for that and lowers the difficulty.

ScottyLems 3 - Square One One of the best levels of the pack! The hardest part is seeing that you need to send the climber out to the right at the start, but once you realize that the rest of the solution comes easily.

ScottyLems 4 - Fight Fire with Fire Open-ended level so nothing hard or special here. I tried going for a save all but didn't succeed. I'm guessing it is possible, but I'm not certain.

ScottyLems 5 - Give The Lemming His Due Somewhat hard but a great level. I especially like the worker needing to bash through in order to float to the bottom to make the splatform for the crowd while the others are contained by a blocker and miner tunnel in progress so the others can't follow. The timing is a bit tight though, but that can easily be remedied by manipulating the RR.

ScottyLems 6 - Groundhog Day Easier than the first 1-of-everything level a few levels prior, but still a nice one.

ScottyLems 7 - Lem Overboard! Another open-ended level that isn't anything too special either, with the only hard part being quick enough to rescue the danger hatch before you lose too many Lemmings. Even that part isn't too difficult.

ScottyLems 8 - Swing for the Fences Another great level! I really like how the two workers need to do different things, like with sending one on ahead to make the rest of the route and the other one making a landing platform to catch the others above the water when releasing the crowd at the end.

ScottyLems 9 - The Acid Test One of the hardest levels of the pack, though that's because I was inefficient with the skills at first and hence was wondering how to overcome the builders which are in short supply. Once I realized the miner from the left entrance was the best way to release the right entrance and hence didn't need to use as many builders there, the level was easier. Nice one, though as Icho mentioned the timer can be a bit tight.

ScottyLems 10 - Sacrificial Lems One of my favorites of the pack! Multiple workers doing different things in separate parts of the level tend to be some of my favorite custom levels!

ScottyLems 11 - The Boardwalk Another favorite of mine! Very clever to contain everyone in the digger pit and releasing them with a bomber in order to get everyone through the trap with just one loss!

ScottyLems 12 - Down The Upside Third favorite level of mine in a row! The entire solution is ingenious, especially with blocking to turn but also freeing him by digging, as well as bombing on the right side of the middle OWW so a faller can fall on its right side to bomb and make a hole.

ScottyLems 13 - Cream of the Crop Nothing too special here either, especially as the save requirement is very lenient and that the hardest part of the level is just knowing the route to take.

ScottyLems 14 - The Dugout Keeping the flow going is the name of the game here, where it's just assign floaters and building across each gap you get to and then just releasing everyone at the end with a digger and make anyone who got trapped at the bottom a climber.

ScottyLems 15 - Stuck in a Rut Another favorite of mine, even if I figured this one out pretty fast! Bombing so you can bash under all the pillars at the start is nice, and also being able to mine the crowd out that is contained in the pit at the bottom. The rest of the solution is pretty nice too. 

ScottyLems 16 - Lemmings on the High Ground Another excellent level! Very nice use of the miner to turn the whole crowd and keep them stuck in an endless loop while one goes on ahead to do the building to get out of the pit and then get across the gap before the exit. Well done!

ScottyLems 17 - Trifecta Another favorite even if I figured this out in about 30 seconds shortly after entering the level Nice use of the digger so one can send a climber out and over in order to breach the middle OWW.

ScottyLems 18 - Kessel Mineshaft Another great level that looks hard but isn't since compression through the traps is easy to achieve with the entire crowd. Probably the hardest part is maxing out the RR to get through both 10-ton traps and figuring out how to get the crowd up to the very top near the exit, but that's just zig-zag miners.

ScottyLems 19 - Out on a Lem Another great builderless level! This level kind of reminds me of a level near the end of the Taxing rank in Deceit's Lemmings, where the solution is more or less the same as this one. I wonder if you got inspiration from that with this one? I think it was builderless too, but I can't remember ATM. Nice use of the basher to make a turnaround point for the other two athletes so one can climb back up to the top to dig down twice and get into position to free the crowd, and for the other to bash through the big OWW to open up the path to the exit. Well done!

ScottyLems 20 - Deus Ex Lemina Another great level with using the last Lemming from the right entrance to bomb so the everyone from the left entrance can climb out, where then the penultimate Lemming needs to bomb in order to have the final Lemming float down to build to catch the others from above once you release them with a digger. Excellent!

ScottyLems 21 - High Rise Lemmings Not as hard as it looks despite appearances. Nice level of sending over three climber workers over the top in order to block so the second can turn around and build to the exit, while the third one bashes through to release the crowd. Then the final Lemming releases the blocker by bashing and then uses the right exit by climbing.

ScottyLems 22 - Rapid Descent Not a difficult level itself, but only hard because of the strict time limit. Even then, time isn't too big of an issue. Nice level.

ScottyLems 23 - Separation Anxiety One of the hardest levels of the pack IMO. It was difficult to work things out here, especially with separating the Lemmings into two groups, with the left side Lemmings being climbers to use the left exit, while the right group being the floaters to use the floater only exit on the right side. The thing that especially threw me off is how you need to send a climber over on the right side to mine the floater group free, because I wasn't sure if I be able to save enough. Nice level, but wow, hard one.

ScottyLems 24 - Keep Your Lems Close... Another level that isn't hard itself but has a strict time limit. Even then, it can still be challenging to work out where to use each skill since the toolset is quite restricted. I'm sure there are ways to make the time limit less of an issue so that not a lot of time is spent walking or taking long detours though. Save requirement is a bit on the lenient side, so that drastically reduces the difficulty.

ScottyLems 25 - Down to the Wire A level that looks hard and imposing but isn't despite appearances. I had my struggles here at first due to overthinking, but once I realized that you don't have to build from both sides of the gap at the same time, then the level is easier. It can still be challenging to work out the finer details of the solution.

ScottyLems 26 - Vertigo Definitely the hardest 1-of-everything level of the pack, though that's justified due to being near the end of the pack. I was too focused on figuring out how to turn around the worker back to the left to bash through the pillar at the bottom, but once I realized that the stuff on the left isn't decoration, then the solution came to me easily. Excellent level, one of the best of the pack too!

ScottyLems 27 - Open Season Definitely one of the hardest levels of the pack. It wasn't easy to figure out how to release all entrances and same thing with isolating a worker to make the path, especially in the middle where the OWW is. It can also be difficult to ensure that Lemmings aren't too close to the one that goes on ahead to build over the bear trap so that you don't lose anyone, but fortunately you're allowed 2 losses instead of one if you don't have the timing right. Great decent challenging level nevertheless!

ScottyLems 28 - Re-Use Miners & Climbers Easy one, with the only hard part timing the second climber correctly in order to turn around and mine the crowd out at the bottom once you release them with a miner on the left side where the entrance is.

ScottyLems 29 - Aftershock Not a difficult level, but not an easy one either. At first I had the wrong idea of building over the exit, as that caused me to lose one too many Lemmings. So, I went through the thin pillars below the exit and was able to spare a builder in the end. Nice level.

ScottyLems 30 - Upsidedown World II Nowhere near the hardest level of the pack by a mile, but it's still a challenging level to finish off the pack. You're very limited on bashers and miners again, with your most prominent destructive skills being the diggers and bombers. Getting through the web with climbers and bombers is quite tricky. You fortunately have plenty of builders to get back up again after using bombers for the web. It is important to save the blocker for the very end so the others can turn around towards to the exit.