Monster crystals are acquired by combining 100 etc drops from a monster in a certain level(Maplestory powerleveling) range. Hover your mouse over the monster crystals you need in the ingredient screen to find out the right level range. You can also acquire monster crystals by disassembling items.
Note: You can only make monster crystals from monsters that appear in your monster book. Nothing from Singapore or Zipangu can make monster crystals. Your item the particular colour. Dark Nisrock? Black powder. White Nisrock? White powder. You get it by now. Magic powders have about the same drop rate as ores, so use the maker skill item section to find what monster drops the colour you need.
How do we disassemble an item in Maplestory?
Open your maker skill window. Place the item you wish to disassemble, and work your magic. You can only disassemble items that can be made. Disassemble anything else, and you get nothing. Disassembling also only gives you back monster crystals. Stimulators, gems, and other ingredients are lost.