Despite the insane ratio, this achievement is not very hard, especially if you have already cleared the base game and learned its mechanics. In Score Attack mode, you will have an unlimited amount of time as well as unlimited continues to surpass the High Score which the game has listed for each level. Boss levels are included.
You will notice in the upper left-hand corner of the screen that gems are replaced with a score multiplier during this mode. As you collect gems you build your score as well as the multiplier (to a maximum of x10). The multiplier drops rapidly and does not increase unless you collect more gems.
Defeating enemies also awards points, and this is how you farm for High Scores as needed. Most enemies only give you points the first time you kill them, even though they respawn. You will want to keep your eyes open for flying green owls that respawn, and defeat them quickly, preferably while you have a score multiplier. The green owls are placed within the game as platforms to help you reach otherwise unreachable areas. You will notice your points continue to build by defeating them over and over.
Almost every level has strategically placed green owls which you can farm infinitely for points until you hit the magic number needed for that level's High Score. You won't need to abuse this technique too much depending on how fast you can collect gems and if you know where the hidden caches of gems are located. I personally did not find any single level to be particularly challenging when it came to reaching the High Score. The owls basically allow you to pad your score, ensuring you don't fall short.
Make sure when you enter each level you know the score that is needed and do NOT approach the level exit without that score or you will have to do it all over again. I find World 3 to be the hardest no matter what the focus of the playthrough is, as it has the most, longest and trickiest levels.
This achievement can be time-consuming and repetitive but it's not nearly as tough as clearing the game on Hardcore mode or some of the other achievements in the list. It would probably fall into most gamers' Top 10 Achievements by TA Score and Ratio so go get it! Feel free to send me a message if you have any difficulty. Happy Hunting!