Update 20190708: Completely redoing this math because it was wrong.
DW3 fighter crit chance is (1/64) + (63/64 * level/256).
Alena's crit chance is simply (level/256).
With Stiletto Earrings, she hits twice. The odds of getting at least one crit (to kill a metal babble) are:
(level / 256) + (level / 256) - (level / 256) ^ 2
2 * level - (level / 256) ^ 2
level / 128 - level^2 / 65536
@ 20 : 0.150146484375
@ 26 : 0.19281005859375
@ 30 : 0.22064208984375
When does she crit more often than the Demon Hammer?
-1/65536 (level^2) + level/128 - 1/3 = 0
level = 46.977 = 47