Last in, First out is by far the best option:
A) no matter what’s the number of redundancies, those at the bottom would want to come back when hiring starts again, and are cheaper to re-qualify.
B) you really need to keep the senior staff, especially trainers to be ready to do the task in A above.
C) if you offer voluntary Redundancies, you have no idea who’s gonna take it, so you could loose all in B above.