Oh, by the way, an answer to my own original question: numbers seem to confirm that the effectiveness of CB is exactly
- 100% for normal and magic enemies (that is, full 25% and 100% with CF(6) which is x4)
- 50% for rare enemies (thus 50% of 25%, i.e. 12.5%, with normal CB)
- 25% for epic enemies
- 12.5% for legend enemies
- 6.25% for mythic enemies
Each level is half of the effectiveness of the previous one. This doesn’t mean it takes twice the time, but actually it needs a lot more because at lower HP the effectiveness drastically decreases.
An example: while a magic enemy gets one-shotted with CF(6), a rare enemy will only lose half of its HP each time and - until you are capable to deal the remaining half with another effect - CB won’t be able to kill it at all.