Achievement Support: Where do consumers reach out for missing achievements?

Sounds like an oversight in development.

Either, they expect another achievement to have existed but didn't account for it in the Xbox version

OR

The 3% missing is the achievement you are waiting for itself and can't unlock because of that missing 3%.

Might be worth reaching out to NAMCO BANDAI Support to see if they are aware of such an issue -
www.bandainamcoent.com/.../support.html

Heya TealRedeemer.

While you can check True Achievements for advice on unlocking Achievements, I don't recommend reaching out to them for support. They don't have anything to do with the Achievement unlocking process, apologies for any confusion with that.

So long as you filled out that survey the right team should be notified of your trouble.

Hello xXToXiC1132,

This forum is for Xbox Live Rewards issues. We do not handle gamerscore/achievement issues. You may wish to reach out to Xbox support (http://support.xbox.com/contact-us) for issues with achievements unlocking
or gamerscore.

Suggestion to earn this achievement: Play a big team battle-slayer game. Map = Spire Get the rocket launcher (there are two of them on the map). Stand near the cliff and wait for two enemies to get in a Falcon. Lock on, shoot, and jump off the cliff. Wait
for about 5 seconds and BINGO - achievement pop after the double kill. Of course there are many ways to do this, but this way seems to work every time. Also, reviewing that actual Halo forums may help with Halo related questions. Good day to you, and hope
this advice helps!

Like everyone else suggested, please try again. This is not an issue with your Xbox Console - it's either a glitch on Halo Reach or you may have not fully completed the task given in said achievement.