XXI - Blood Astronomia Achievement in Vampire Survivors

All of these achievements that start with a roman numeral CANNOT be unlocked until you obtain the Randomazzo relic. Refer to Randomazzo (10G) for doing so. The original Steam achievements specified "Reach Level xx with [character] after obtaining the Randomazzo," but for some reason the final bit of the description was removed when the achievements were ported over. You therefore won't be able to unlock these achievements until you progress through most of the main levels, unfortunately.

As far as reaching level 50 with each character, by the time you reach Stage 4, Gallo Tower, and get the Randomazzo, reaching level 50 should be a relatively simple task. It can be done on any stage with the character in question, so my recommendation is to use Inlaid Library. Enemies only come from left and right, making it far more manageable. Once you get the hang of it, you should be able to hit level 50 by the 15-minute mark. As I've mentioned numerous other places, grabbing the evolved Santa Water, evolved Axes, and evolved King Bible will make it very easy to survive in this stage. However, if you're grinding out these achievements towards the end of your completion and are therefore fully upgraded, weapons unlocked, and know what you're doing, you should instead grab the Crown and Attractorb as soon as possible to make these achievements go as quickly as possible.