Marble Blast Ultra

To play on the Spires map you'll first need to download the Agoraphobia map pack which currently costs 200 MS points. Once you have the map, the easiest way to get this achievement is to host a player match with a 2 person limit and one private slot. This will allow you to play on the map by yourself without any competition and will still unlock you the achievement(thanks to Tallboy2005 for the tip). Also note, you need to collect 50 gems in ONE GAME. It's not cumulative.

As for getting the gems, that can be a bit tricky on this particular map. The easiest way I found was to use gyrocopters. Try bouncing off walls when using the gyrocopters if you need to gain more height. If you're still having trouble, try activating a gyrocopter and stay on the ground. Then roll over to a super jump and use it before your gyrocopter runs out. This will give you much more than enough height. Now you just need to land on the gems. Be sure you aren't spinning your marble too much when you land or you won't be able to stay on whatever platform you're landing on.