This dungeon first appears around level 26 and I believe it disappears at 31.
All bosses including optional ones need to be defeated for achievement to pop. I have had 1 maybe 2 dungeons complete to where I got the achievement before the optional boss was killed
As soon as all the dungeons appeared I stopped questing until I got the achievements for the available dungeons before they disappear. Questing and running dungeons and doing the "Vanguard Requests" will make you level pretty fast and I do not know if you are to que up for them later in the game. I do know that you are able to enter each dungeon(at the dungeon entrance) solo or with a premade group you have to put together yourself. I have heard and read(have not tried personally) it is recommended to be above 15-20 levels with gear level appropriate to your current level before trying to solo a dungeon.