I've reach Theseus and the Minotaur three times now (well Theseus wasn't there the first time for some reason...), and got stomped each time. Decided to switch things up a bit, so I bought back all my purple gems from the mirror and used them to get 3 x Death Defiance instead, so effectively I now have three lives.
Currently about halfway through my next run, and managed to get halfway through the fire level without using any of my lives. Looking good!