This isn't detailed but I feel it may be useful for people, it's levels where badges are useful, and other little titbits. It's a bit out-dated but when i'm back to this game very soon I'll likely update it maybe

If you have a free badge you should use:

1-5 - Pit badge 1-6 - Big eggs if you want but not really important 1-8 - Pit badge 2-1 - Big eggs 2-3 - Big eggs 2-6 - Lava badge 2-8 - Lava badge 3-3 - Pit badge 4-7 - Speed I guess? 4-8 - Watermelon (unless you aren't using 2p strats on this level, in which case this badge is compulsory) 5-1 - Speed 5-2 - Speed 5-4 - Big eggs 5-5 - Speed 6-1 - Speed 6-2 - Speed 6-4 - Speed 6-7 - Speed 6-8 - Speed (though you should be able to afford this anyhow)

Compulsory badges (if you find you have enough money for extra badges after these use them as per the freebie list but make sure you keep enough for compulsory stuff):

3-8 - Pit 4-4 - 2x Poochy at start/in level 4-6 - Speed 5-3 - Pit 5-7 - Pit on P1 6-3 - Watermelon

Levels with 2p strats:

2-2 2-3 (though it's really really hard) 3-2 4-8 (ground pound strats on boss) 5-4 5-6 5-7 5-8 6-5 6-8

Again, this is a little out of date. I'll update it when I'm back maybe