Vampire Survivors

Each of the five stages has a coffin to open, and they all work the same way aside from Gallo Tower's (which I'll discuss below). First get the Milky Way Map before attempting to open any coffins. That way, when you start a stage, you can pause and see the coffin marked on your map (it's the big question mark). Then, you'll need to work your way to the question mark. When you get to the coffin, a ring of enemies will spawn around it. You must kill all of these enemies to open the coffin, not just create a path through them. You should therefore level up a bit before heading to the coffin, just to make sure you can quickly kill the surrounding enemies while regular enemies are still swarming you. Once all the enemies are dead surrounding the coffin, walk over it and hit it to open it up and obtain a new character (and the associated achievement).

The only exception to this is the coffin in Gallo Tower. As you head north to the question mark, it'll be off screen out of bounds to the right of the level. To access it, just south of the question mark, on the left side of the map, is a silver mirror hanging on the wall. You need to walk up and into this mirror. You'll be transported to a small side area where the coffin is. Head down to spawn the ring of enemies around it. Now you need to quickly kill those enemies, because you don't have much room to move around, and they gradually pulse outward farther and farther away. It's possible for them to get too far away such that you can't reach them if you take too long to kill them. So, definitely level up a bit before heading through the mirror to ensure you can quickly take out the coffin enemies before they spread too far apart. Then, just open the coffin like normal.