Why do I sometimes take massive damage out of nowhere?

You are in the game for such a short time and you are at level 92 already fighting Red Maps.

This is quite an acomplishment my friend, at least shows that you can understand alot, but as I point out before you have to understand game mechanics and your flasks.

Bosses are not designed to be trivial to ALL builds after some level, and yes some builds are doing fine against some bosses where others strugle. I bet, that if you want to continue with this game and choose another build next league, you will find that probably some other bosses now are giving you a hard time (try to fight acid lake boss in a blue map with a melee build or even canyon boss with bad mods and we talk again).

To retreat and not to fight a boss is not a shame but some times a necessity (like you don't run maps with lethal mods, cause simply you or your build can't).

Yes there are some OP builds out there, that trivializes everything in the game, how easy to gather all the artifacts for them, ADAPT your playstyle to them, and pray next league not be nerfed is another story.

There is no better defence than Predict and Move (by running again and again and again the same boss).

And yes everything come with a price (you can endure a hit, but with low DPS the fight is long enought so several hits eventually will hit you and maybe one of them is the fatal one, Hight DPS with crit short fight BUT the first hit is the last one).

I always get acrobatics and phase acrobatics as a ranger.
Do you think that I stand in Aztiri's flameblast and doing nothing, cause I spend 5 passives and have some spell dodge?
I move like crazy and only ONE time the dodge mechanic works.

Do you think that as a Jugger with extreme regen and thousands of armor/life I stand still?
I move as a maniac and boy the boss fights are realy long (that's why we have portals).

And yes "...sometimes shit happens man".