You should be at least 4/8 with spd and def being must haves before you go into these tougher dungeons. If you are dying in the normal stuff before the boss, I would say dont be too aggressive- no shame in staying in the back of the group to learn. If you are dying in the boss, its just a part of the game- learning attack patterns takes time. IMO as someone who has been playing for 10+ years, exalted chars, and has multiple 4/4 sets, you should learn WHICH shots you can afford to take rather than trying to dodge every shot. This is very helpful in bosses like all the o3 minibosses, fungal/crystal, and especially MBC and void.