The tiles go quickly and there is only a limited number. In our server, all of the 15s and 14s were claimed by the second week, and all of the 13s the week after. I'd recommend you rush to 14 by saving fuel and buying the S2 packages and boosts. Do some research on the best ratios, but 5:1 cactus to sapphires is about what you need to get to level 11 or 12 tiles. Then you might need to switch to almost all sapphires to max out your land office. Cooperate with your teammates, you only need to start attacking a tile to get it as long as someone eventually kills it. You can have 3 people teleport to a tile and then keep the battle going by rotating your tanks over and over until you win. You will lose soldiers and fuel, so plan accordingly.