This achievement will unlock once you have 24 glyphs- four rows of six. VIDEO
The main way to get glyphs is from building combos by killing enemies. Your combo is shown by a skull meter in the upper right of the screen which will appear after killing an enemy. You have a few seconds to attack or kill another enemy and if you aren't quick enough, you'll lose your combo. Every 5th enemy killed in a combo will drop a glyph so you don't need to get past a five combo- your focus should only be on keeping combo to increments of five.
You can reach the glyph cap in Cave World with the right approach. I used the Brick Head, Scythe, and Shark Fin.
Cave World has four main areas and a secret area accessible in the third area (kill all enemies in the third area and a new world snake will appear). The secret area has numerous flying enemies and a few glyphs. There is also usually a thin upward path in the fourth area that leads to a room with four glyphs. As long as you don't waste enemies by breaking combo too often at non-five increments, you can easily get to 24 glyphs before the Shop.