Earning this achievement is very similar to
but is specific to runes. You will need to acquire the Enchantress from leveling up to buy runes. The first two are available from the start. You will then need to acquire runes from inside the castle to finish this achievement.
To acquire them, you would need to get Firefly chests. They are usually in challenge rooms. Typical challenges include:
-Defeat all enemies
-Reach chest without taking damage
-Reach chest in X seconds
The defeat all enemies is likely the easiest, but some are more difficult than others. For example, there are challenges where the enemies are in locations you can't reach with your normal weapon or you might not have the appropriate spell weapon to hit them. The other type is an open room where there are usually more enemies than normal and possibly a little stronger than normal. This room is where you would want to focus.
Rune costs increase as you buy each one (similar to upgrades) so it will take a little bit. But as long as you are already going for some of the other gold related achievements like:
You should be saving up enough to get there.