Wow… so you saw the picture, and STILL don’t understand what I just said.
Let me draw it in crayon for you.
Thrash isn’t EVER as dangerous as shadowflame unless you have a hunter pull the boss form a long distance. Melee hits for around 1k. Shadowflame hits for 3500-4500 depending on resists.
Half of the reason shadowflame is dangerous is because other abilities can hit shortly before or after it.
In the picture, you see a shadowflame followed 0.8 seconds later by a thrash. The tank should have a bubble on him or a CD just before shadowflame as often as possible. Alternatively you can use a flask, and just out-HP the damage.
FR gear is mandatory unless you’re using some weird strat, so the tank won’t be wearing his best gear in the first place.