Most FATE bosses tend to put stoneskin in their rotation, since it seems you were unable to complete it in the 15min timer I'm assuming the following.

-You didn't use your companion
-Never stunned Stoneskin

If you use a companion it will help your odds of winning a fight, it's definitely easy for a warrior to stay alive however if you are relying on keeping defiance up the whole time that's your third issue. You don't need to keep Defiance 100% of the time only putting it up as needed or when you need to recover HP, that loss in battle potency does make a large impact especially in timed content.

So I'd say if you do end up in a situation where you may have to solo that FATE again or any boss FATE just keep an eye on when they cast stoneskin and stun it, have your companion on hand to add to your overall dmg and only put up Defiance as needed.