I haven't beaten him yet, but here is an advisory on how he responds to attacks:
Tartaros's third phase has this little thing in his AI that causes him to actively pick attacks that he thinks you do poorly at dodging. If he scores hits with various attacks; that weights his RNG into using these attacks more and more. It is through this that getting hit can lead to a swift death, even if you haven't taken much damage before.
One way to play against this is to identify which attacks you do well at dodging and deliberately get yourself hit by them to weight his RNG towards those attacks as opposed to the ones you have trouble with.
Thanks. It seems that I definitely have the most trouble with Death Knight spam in the third phase. I can dodge all of the other attacks well (except for the bubbles, but if I time it right I can run ahead of them without getting hit, circling tightly around Stan so that he doesn't hit me), but as soon as the death knights are summoned (and spammed sometimes consistently), I tend to get hit by everything else trying to do away with them...argh. But thanks, I'll try to take that advice next time I attempt him