For any still having issues with this we found the easy step was keeping one ghost on agro which prevents it from reseting if you are slow to engage between bosses.
Choice of kill 4 bosses at the same time then burn the splitting mob. Or take all others down to 5% then kill the splitting mob and finish off the other 4.
Choice of kill 4 bosses at the same time then burn the splitting mob. Or take all others down to 5% then kill the splitting mob and finish off the other 4.