I checked the wiki seems dark is your best option because phantom union give another 2 dark weapons. So you will have the strongest headstart because you will have 4 union event faster than any other element. To get this best start you must start it now and find decent union so you can get union event rewards. 3 best for f2p are dark light and wind. Those give best flb union event weapon. You missed the light so dark and wind is your choice. As long as you can do bit extra effort in leveling in sunday and doing union event daily just 1m should be very easy to get.