So, I just fought Stormbringer Karanda with a level 61 Succession Valkyrie, TET Blackstar, couldn't run out of the red circle in time, put up her shield, but she gets stunned, drops her guard and dies.  She gets stunned in guard mode on normal Karanda's small red circle attacks as well.  So........ why even have a shield?  Sure, Valkyrie can escape with her dash ability, but.... why have a shield at all if the point is to dodge?  Should EVERYONE fight bosses with Archer?  Which IS tons of fun, by the way, but STILL.... this game has TONS of variety: why can't it all pay off?  Why does the strategy have to be the same?  Or is the strategy: don't fight awakened bosses until you're PEN Blackstar?  I guess... but, I seriously doubt even that can stop an awakened boss's attacks.  But in the chaos of melee, it can be hard to detect the oncoming attack in time to dodge, so if you have a shield.... why can't it work?  Maybe I'm too unskilled, have crappy gear...., but I think the answer is: Sorry, dude, wrong class for fighting bosses.  Try Archer.

And seriously, try Archer if you haven't.  He's awesome. 

And Succession Valkyrie should put her shield away and try to be a Lahn, or something.....

P.S.  With recent inflation rates, Boss loot is pretty much worthless.  Perhaps it could be revamped?

As if a tiny arm shield will block a huge 10 story bird from hell.  Everyone gets one shot at Stormbringer / Nightmare.  I made the mistake of taking my tag Striker to Nightmare Kzarka with 395 DP at the time and got literally one shot and lost a JIN Harphia and JIN Viper in one death ~_~ just how it works.

Ranged classes is probably META for them, IDK.  But my 2nd Sorc is parked at Karanda and my Musa is at Kzarka, they both do normal and awakened version of the bosses.  Just dodge when you need to.  Don't sit in red circle for Stormbringer / NIghtmare.  It's how the buffed bosses work. 

The answer is: Red circle, get away.

because she sexy with her little shield and her little sword ^_^

Devs really should rework PVE AoEs.

You can block some of them, and some you can't. It's just a guessing game or trial and error.

learn to dodge, some boss attacks ignore protections

valk has that sideways spammable "iframe" thingy. not the solution u asked for but.... :D

they dont ignore iframes, which kinda puts classes with other defense options at disadvanage :)