A Boss Battle will be noticeably more difficulty than even the blue Elite battles that can appear and will occur with fixed frequency, that being once every five stages. Therefore, there are 3 on a single run on Transcendence 0. This means that if you are winning each run, it will take 6 2/3, though it would be possible to beat the first one 20 times if you felt like you wanted to. This will come naturally over the course of getting other achievements, as it would be unwise to stack the runs with specific condtions, like No Items, Defense Cards or Skills (unless you really fancy a challenge).