I've just completed the pit solo but didn't get the achievement. I did this over a few sittings and apparently a lot of players have had the same issue and found that you actually have to complete the whole dungeon without leaving once for this to trigger. This must be a bug - I'm sure shattered throne didn't work this way so how are players meant to magically know the same triumphs work differently across dungeons? The triumph states nothing about this so the only (reasonable) conclusion is this is indeed a bug. Bungie, please fix this and credit those affected by it.