TY the Tasmanian Tiger

In a few quests, some characters will ask you to help them, and a option for saying "Yes" or "No" will pop, the first mission that does this is Rex's mission in Bridge on the River Ty, just say No to him (or any other character that gives you the option to say Yes or No) and you'll get the achievement. Other missions that give you this option is Rex and Elle's mission in Rex Marks the Spot and Ranger Ken in either Outback Safari or Beyond the Black Stump, don't quite recall.