Have a saved game file from the demo version of Gundam Assault Survive. Create a character. Go to the shop and select the "Secret" option. Purchase the item that costs 0G to get 250,000G.
Have a saved game file from Gundam Battle Tactics, Gundam Battle Chronicle, or Gundam Battle Universe. Go to the shop and select the "Secret" option. Purchase the items that cost 0G to get 100,000G for each of them.