1. The achievement description was copied over from the Windows 8 version even though there's 2 additional difficulty stars now. You need to complete all 6 stars.
2. There's a collection named 'Kids' that makes this much easier thanks to having 190 pieces on the highest difficulty versus 600+ pieces in other collections. It costs 3400 gold and has exactly 10 levels. If you can't find it in the collections screen, you can make it appear again by deleting: C:\Users\USERNAME\AppData\Local\Packages\Microsoft .MicrosoftJigsaw_8wekyb3d8bbwe\LocalState\Download edPuzzles\KidsPackPack
(USERNAME stands for your system account's name)
This should be unlocked naturally in your first or subsequent Skirmish playthroughs.
You can make things a bit easier using cheat codes if you want but it's really not necessary. Enter speed always wins (for instant build/training and resource gathering) into the chat box (press Enter to open the chat box).