This achievement does not have to be done all in one attempt, and can be done in multiple visits. It is almost not possible to do in one attempt since if the raid takes too long to gather up all the dwarfs and get to 25, Razorscale will enrage.
To do this achievement, the raid must weaken down the adds to about 20k health. When Razorscale is brought down, the raid must then position all the adds in front of Razorscale so when she breaks out of the chains, her fire breath will hit all the adds and kill them. The tank will need to be healed through the fire breath as well.