These achievements require you to obtain a score of 50,000 in Endless Mode. This mode consists of a semi-random never-ending array of obstacles. You get points based on how far you get before failing. Reaching 35,000 is not terribly difficult, but making it to 50,000 will take some practice for sure, especially since the levels are random and cannot be memorized entirely.
A good method for this is to wait until you have all other achievements (that you actually want to get), then reinstall the game. You WILL lose all your progress, but you will get your free 3 slow motions replenished. It is significantly easier to obtain these using slow motion. If you want to use all three in a row, you just need to legitimately get to around 15,000 points, which is not that bad. Once you reach that score, use your slow motions consecutively. They will take you far beyond 50,000 score. The only tricky part to this method is making sure to activate the subsequent slow motion immediately after the previous one has expired. It is hard to tell exactly when this has occurred, as the fade back to normal speed is slow. Just keep jamming it when you think it is close, still paying attention not to mess up. If you do fail, it is still easy with two slow motions (since you wasted one on your first attempt), however you will need to get to around 25,000 points before activating them this time. You could also just reinstall the game again. Endless Mode is available after STARTING Story Mode for the first time and your slow motions will be replenished.