Time monsters come back during a short period of time. When you kill a time monster and suck up its Time Crystals you will get money. Wait for Time Monsters to come back, then keep killing them. You will gain money every time you do this.

As a Timesweeper, get a Sweeper that can have the Time Crystal limit increased. Upgrade it completely, and if possible get a secondary and do the same thing. Enter the first round. Go through missions 1 and 2 quickly. On mission 3 (fighting Stone Guardian), ignore it and run around the island collecting crystals until you cannot get any more. Then, fill the Sweeper with as many bombs as it can hold. Kill the guardian and you will get approximately 3,500 to 4,500 for the one mission. Repeat this as may times as desired.