P3's leader always gets attacked by the missile at the start. Only use physical attackers, preferably sword users (Bahl/Zuzu) and no spear users. Ellvern/Zafitte are amazing because of Antidote/Levitate. Kill that wolf thing that casts weapon damage absorb shields on the boss. After doing Artemis 1 and 2 so many times I don't think it seems too hard, but people bring bogus characters to the fight, so I haven't actually won yet.
I just beat it in a public co-op. Started the room myself and left it until I got 3 people with decent characters (gugba, Sheena, Zuzu). I myself rolled with Amina (survives quite well), Zuzu and Piz'fer (also survies longer than any other of my chars + has a self revive).
You definitely need to clear out the poison monsters first or else you might be seriously screwed when the boss locks on and you need to move a lot. After that it was somewhat easy.
The third person had 2 of his chars dead and eventually disconnected and I eventually beat it with the other guy. Man, was it intense...at the end of the battle I actually felt like I bonded with the other guy, haha.
You're my bro Ichiryuu
Edit: It's also important that the first player's first move is to make sure the third player doesn't die from the initial missile attack (since they will get attacked again). That seems to be the main cause of the wipe...either that or the third player not knowing how to secure everyone away from the lock-on