I was in TB today i touched the boss and received 15k damage on the final boss. I asked another DK who was the same level as me in the raid to test it out and he also received 15k damage, the difference however was that i had 15 points in achilles where as the other DK had 0 points. I disconnected and went to the FM to swap some points around and made my Achilles from 15 to 30 points. i went back into the raid and touched the boss and still received 15k damage.
The damage shown on screen is not the damage you actually take. The 15k you saw reflects the damage before achilles took effect.