Leave this achievement for last. Once you have all of the others you will have surely completed several Bonus Objectives. Then play Amazon Level 3 over and over again. Each race takes a minute and as long as you come in first you will get an optional objective completed. When you completed 3 of 3 it rolls over to 1 of 3 and you can keep completing them.
This is the criteria for each of the three Bonus Objectives.
1) Never let the bronze opponent beat you to a checkpoint.
2) Never let the silver opponent beat you to a checkpoint.
3) Get a silver medal without dying.