Just focusing on the boss and ignoring adds will probably work in normal and maybe heroic depending on your gear. I've killed serpentrix normal with my 5 hunters by just nuking the boss.
With my 5 paladins I've killed serpentrix on mythic, there we have to kill the adds I believe. Posted a video on this yesterday.
Anyway the one thing which makes one survive that fight is to be on the move all the time. Most of the damage taken in that fight is by things which is cast at a characters current location. So if we move, we can either avoid it, or minimize the damage taken by getting out of it really quick.
So if you have a team which needs to stand still and cast you probably need to be smarter about the movement. The boss selects 1 character which starts spawning pools at that chars location. So you would have to break that char out and move it while the rest of the team are doing their thing.