To make this pretty easy, first unlock the bear mount than start a new game on a big island where is common to find it (islands 4-5) and rush some money for it. On your second day explore until you found the bear (if its not in the map restart the island) and start hunting. Dont worry for the colony, you will make money just hunting deers. While you hunt use the money from deers to recruit people and continue hunting.
Dont forget to always let your bear eat the grass when you can, to speed up the hunting process. When is starting the night go back to the colony and use the money you have made to defend it.
If you feel you will not reach 20 deers you can still hunt at night and drop money on the ground when the monsters come to you, they will run away with the money but you will make tons of gold with deers so dont worry for the loss.
Just to let you know, you have the entire fourth day to farm, the time will expire when the fifth day begins (when you hear the bells with the V roman number).