If you're interested, here is the list of levels I found particularly difficult: (from most to least)
W5L4: Can't See the Forest for the Trees
I was stuck on those boulders for a longest time. Finally the light came on, and I did it. And yes -- I did the final forest by randomly moving in the final maze. Almost sucked out my time, but I found my 20 coins and went to an exit with 2:46 still left on the clock.
W10L10: One last Hurrah!
If I would have a penny for each time I've tried this level -- I would be a millionaire now.
The chompers weren't too difficult, I spend some trials finding out where are the hidden coins and then three more trials to exit with all bonuses... But that UFO/reflector bit really ate my brain. 
W6L6: A Little Detour
I was in a total awe when I've found out how to place all the reflectors. Took me some more trials because I was impatient and got shot easily.
W4L4: Catch Me If You Can
Was solved a long time ago, but it really drove me crazy. Thank god I found some help on the forum.
W6L1: That Sinking Feeling!
The boulders had me so stumped that I looked at the online solution.
W6L3: Bridge Over Troubled Water
I couldn't get the bonuses for a long time, and so AGAIN (gulp!) I had to look at the online solutions.
W4L7: Floating Platforms
For some reason I couldn't assemble this one in my mind for the longest time, and I felt so good when I found out that I can press the pink button.
W10L7: Firestorm
The left part gave me a lot of troubles, and I'm still unsure of how I did it. The important fact is that I finished it with all the gold.
W10L5: There Got To Be Another Way!!!
If you've played this one, aw, you know.
W10L4: Knight's Move
Pretty annoying, but not too bad, once I got past the timer part.
W10L3: It's a Long Shot
I was stumped here for a long time, and still couldn't figure out how to use all the reflectors, so I did it the lazy way.