This Achievement has to be done on level 5 unconfirmed.
This is obtained after you have taken the armor off of a Promethean Knight and a watcher has repaired it. The opportunity for this comes early on in the level.
The Promethean knights are the big mini boss enemies and the watchers are the flying ones.
Easiest way to damage the Knights armor is with precision weapons like the dmr, battle rifle etc. Pop a few shots in each shoulder pad until they explode, then wait for the watchers to repair it (needless to say DO NOT kill the watchers) then repeat and finish it off by shooting it in the exposed glowing section of its back
I did this on legendary without going for it, the chances are if you have taken their armor down they have battered your shield to.
Thanks rich V90 for the info