Can also use Creeping Tentacle instead of Lesser Voidcaller. Might even be better, since Creeping Insanity deals damage one round faster than Curse of Doom. Just make sure its HP is higher than your manafiend, and you should be good to go.
my guess is that your Manafiend is too slow and therefore Shadeflicker is going first and dodges your Surge. The Manafiend must have 246+ speed in order to be faster at step 14 and get the final kill. it could be because its not rare or not 25 (otherwise all Manafiends have good-enough speed). hope it helps.