Does anyone here knows where to farm giant crystal???

repapips

Im having a hard time farming giant crystal.. i use it to upgrade my beast trainer... it requires 50 giant crystal in the initial upgrade but i only got 20 giant crystal and it is given by the gm as myy  in game bonus... when i checked the market it is very expensive costing around 30k per crystal so i cant afford it. so my only way to have giant crystal is to farm them.. any idea on where to farm giant crystal???