MX vs. ATV Alive

Best done in conjunction with the 'on the right track' achievement. set the opponents to rookie and race length to 3 laps on 45 palms national. the important thing is to take it slow. your opponents will not get too far in front of you. the biggest struggle with this one is having the patience to hang around behind first place. as long as you do not go off track at all (the words Off Track will pop up if you do) and do not overtake first place until the last lap you will get the pair of chievos.
NOTE: watch out for first place crashing on a corner, for this reason i found it easiest to not sit in second place or take the corners slow enough to see if they have crashed

EDIT: BwF SmokeBudnSF has pointed out that this can also easily be boosted online. Thanks.

The easiest way to do this, at least for me is to setup a short track race. Any level should work, and set it up as Rookie difficulty and 5 laps.

When you start the race, just keep in second place as best as possible and don't cross the finish line in first until the last lap (5/5) I had to brake for the opposing A.I because they were so slow or would mess up. But on lap 5/5 I pinned it and passed him perfectly fine and crossed the finish line in first no problem.

Hope this helped.