The hidden stage is tough...

If you search on YouTube with the game's name, you'll find a lot of strategy guide videos.

You can see the lower tile check by dragging the upper tile. If it's not connected to the top tile but connected to the lower tile, you can raise both the upper and lower tiles together. For example, if you have registered tiles 1 and 3, and there are no matching tiles on the main board, but tile 1 or 3 is on the lower board, check if there's a connection point with the upper tile on the main board's top. Then, if you register the lower tile that can connect with the registered tile, it will momentarily become a full set and you can register 3 again.

If you do it this way, you can register the same tiles on the top and main board, and proceed while checking the bottom tiles. If you occasionally get a bad layout, you can use hints or use the undo item for all registered tiles to clear the board.

However, if you start using items, you might become reliant on them and miss things you should be paying attention to, so it's best to take your time and look around as you progress. There's no time attack, so it's not too stressful.

Thanks for all the kind replies!!! I'll start leveling up from today.