This is one of the four Dungeons that becomes available at level 60 and probably the last one you want to complete out of the four. It has a Minimum Item Level Requirement of 302 to enter.
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.
Once you hit level 60 best advice is to do "The Vanguard Initiative" quest line. It is about 7 quest long and it will give you full level 60 Initia armor and accessories and good xp and gold. Once you are equipped with the all the level 60 Initia gear I recommend Enchanting all gear pieces to +9. During the quest line you should start to receive Ishara’s Relic Pieces (level 60 relic weapon) these are obtained by killing monsters in Velika Outskirts and the zone "Vanguard Requests" quest when you’re level 60. I recommend enchanting to +9. Once that is complete you should be well above the requirement for this dungeon. Just remember to make sure you knock out the other level 60 dungeons first, for the achievements as they may disappear after a couple levels. After those are finished you will grind this dungeon(or quest) to level 63-64 when that last dungeon that is tied to an achievement becomes available which is the "Macellarius Catacombs dungeon". Grind the Sabex Armory dungeon(after completing the other 3) the bosses here drop your next set of gear which is the Emina gear and accessories which are level 62. This level 62 gear will hold you over until you hit level 65. The Macellarius Catacombs dungeon will have a chance to drop your level 64 weapon.