As your armies get larger and your units get higher in level, it will take more and more downtime between battles (unless you want to spend gold to speed up construction of units (which I do not suggest: use the gold to help pay for the expensive upgrades - such as the obscene costs of Command Bunkers or leveling up units). Getting to 2,500 dog tags will take some time. This is much less of an issue since you can no longer target specific accounts (as I leveled up to 2,500 dog tags, I had to keep my alt account around 1,500). Once you are over 2,000 dog tags, you can no longer attack anyone at 1,000 dog tags.
You can only attack and be attacked by people who are within 1,000 dog tags of your level. If you have 1,200 dog tags and are level 35, you can be attacked by someone who has 1,300 dog tags and is level 140. Match making is done based on the number of dog tags, not on your overall level. I'll recap some of the points in case people skipped here looking for this achievement: Once you have a total of 45 stars (and this can be 15 missions completed with 3 stars each or more missions if you can only get 1-2 on certain maps), you will get: For example, if the reward is for 10,000 green crates and you manage get 4,500 on your first attempt, there will only be 5,500 green crates left when you attempt the scenario again. Also, the reward for beating the scenario does not replenish. This means you can take your time and don't have to dump all your forces in to beat the time limit (which means less rebuild time between battles). Like with the multiplayer battles, you can get a maximum of 3 stars per battle.
You will need 45 stars in the campaign mode. For me, it was the next-to-last achievement as I was saving up the funds for rank 10 supply depot.
You can certainly get it earlier, but the longer you wait, the more and stronger units you will have which obviously makes it easier. I'm adding the achievement for getting stars in the campaign in the grindy achievement section.