Tag CollectorG 15

Tag Collector G15:
You have collected 1,000 Dog Tags.

This achievement can take a while I just repeatedly do Operation 2 Mission 3. Grab the dog tag at the start, then follow the river. There will be 2 more tags. Cross the bridge for 4 more dog tags, then pick up the one to the right of the bridge and 3 more straight back from where the bridge ends. Then abort the mission. Takes 30 seconds and you get 11 dog tags per run.