Wish we could help but we can't go back and even see what challanges look like that we've finished. Try looking on Youtube. In general, though:
- Avoid spending all your minions at once. It's a terrible habit that will get your whole army killed when raiding so it's probably not going to work well in a Challenge either.
- For most defenses, especially ones like poison traps, training room, and hatcheries which don't have a limit to the number of minions they can damage simultaneously, avoid using more minions than are absolutely necessary to eliminate them. Conversely, throwing 20 trolls into the workshop or dark library is the best way to reduce the amount of damage it does.). This saves your minions taking more damage than necessary. Believe it or not, for example, but 1-2 skeletons and 1-2 trolls can take down a hatchery almost as quickly and far more safely than 10-12 trolls alone.
- Send one skeleton ahead alone if you aren't sure there aren't traps ahead.
I'd give more specific guidance if I knew what the map looked like. Maybe Google image search?