I'd recommend for this achievement you be at least a past level 10, the first boss battle recommend level is 25 however I joined a lobby as a 32 with two players at level 11 and 3 and we managed to defeat the boss anyway.
For this achievement your want to go to the Governors Office or lobby if you use the teleport in town. Once it loads you in, look to your right and your see a kiosk desk, go over to it and interact with the NPC. From here you can create your own room or simply join a lobby that has a open spot. Once your in the lobby with others, you can check your equipment and skills using the small blue screens on either side of the lobby next to the kiosk. If your hosting you can interact with either sides NPC to choose a quest. Once your set and the host of the lobby chooses the quest, you press
and hit ready. From here your be stuck in this menu until everyone has ready up. Then complete the selected quest and the achievement will unlock.
As for the boss battle itself, keep a eye on your health and watch out for the attack patterns of the boss. I've not failed any of these and a team of 8 seems to defeat them pretty nicely. At the end of the quest your be rewarded with items that are set. This might be a good way to farm for a certain item if you need to.
Also note there is a separate achievement for defeating 10 different bosses in Co-Op. So your want to work your way down the list for that one to.