Cosmo Run

This is probably the hardest achievement in the game, especially at first when you're struggling to even get to 100. The best way to handle this achievement is to first not rush it. You will be playing tons and tons of runs to grind out the 200,000 score so you'll have more than enough time to obtain this achievement. It will be very tempting to use Save Me a bunch of times to get to 666. This is an option, but you are just going to waste all those stars. You are only going to get better as you play more anyway. Here's how I recommend doing it:

Get all other miscellaneous achievements first and by that time you should be able to perform decently well and at least clear 100 without issue. Keep practicing (and grinding) and you'll get farther and farther. You should also be collecting stars and saving them up. Once you're finally able to get up to around 300-400, now is the time you can start using Save Me to get you that last extra bit. I highly recommend having around 1000 stars at a minimum when you attempt this, just to be safe. You shouldn't have to use Save Me more than twice to get to 600. Once you get over 600, I highly recommend purposely letting yourself die when you get to around 660. Get your bearings, calculate how many more points you need (6 turns, for example), use a Save Me and then count the turns so you end exactly on 666. Purposely end the run then and the achievement will unlock.

It will be tempting to try for this early using Save Me often. I used Save Me twice to get it. But to put it in perspective, after getting this achievement and grinding toward 200,000, I consistently get over 1000 without using a single Save Me. So you will get better. Just be patient and wait until you can consistently get close before using Save Me a bunch of times.