50 reincarnations is very easy to get, but it just takes a long time.
Pride challenges help achieving this because of the permanent stat rewards, but make sure you can finish at least the first three endings because pride will become difficult pretty fast.
If you really need to grind for this achievement, go for 'Willing Revenge' and skip the parts with Norn. This speeds things up significantly. Graduate as soon as possible and go for the ending. Each round should take much more than 2 hours, if you really focus you could probably do it in 1,5 hour. With really high stats you can even avoid a Dark Ritual all together and just go for under an hour.