My PotA Arcane Trickster just nerfed a boss fight:
My group was supposed to be sent to the Tower of London and quietly assassinated. Of course we turned the tables and survived. The boss executioner had distinctive magic on his body. When we saw HIS boss coming to check on progress, I used Disguise Self to look like the dead guy, walked out, and reported "We succeeded but they dealt me a blow (hands to throat - no illusionary voice) and we still have to clean up." The boss thought about it, said, "OK, well done." and walked back to his office.
So the enemy never struck a blow, neither did we, and we accomplished a strategic success: left alone long enough to escape.
Next step in the plan: when we come back for revenge, I'm going to disguise myself again as the executioner. If anybody challenges me, I will say "The rumors of my death were greatly exaggerated ... but I'll let them swirl around, because nobody will expect to see ME when I show up to deal with the problem."