There are boxes all over the map marked on your radar as a white icon. Every one you collect is worth 10 resources; however, death will cause you to drop half your resources on the ground for enemies to pick up. The achievement will pop as soon as you have at least 200 resources. No need to end the game with that many.
As far as strategy goes, I would recommend spending the early parts of the game collecting as many of the 10 value boxes on the ground to try to populate the map with as large of a number of resources as possible. Keep doing this until there are 3 minutes left. When the game warns you there are three minutes left---then you should try to kill as many of the other players on the other team as possible to get all their resources in an effort to climb to over 200.
EDIT: As pointed out by others in the comments --- sometimes the achievement does not unlock after satisfying all the requirements. Try closing and restarting the game and that has appeared to work for some.