Elektra wins the fight. The skill gap (if there even is one) is very slight, and nullified by Elektra willingness to kill. In a straight up confrontation Cass will be on the defencive right off the bat (even if Elektra is unarmed). There is also the fact that Elektra has some low level telepathy and creat complex senerios in the head of her opponents but I'm not sure if it would work on Cass as it has never been used on someone above your average cannon fodder... still a wild card though.
I see Elektra taking the slight edge in a straight out confrontation, and in all likely ness she would kill Cass with the greatest of ease in a surprise attack... chances are she could assasinate her with out Batgirl knowing she was there.
I think Batgirl could take her in a few panels. She's faster, tougher, stronger, and more skilled.
Elektra's not necessarily better than Bullseye. She didn't look too special in their last fight and would have died again if Daredevil didn't catch the card thrown at her throat.
So girl who can dodge point-blank shots vs. assassin who can't even dodge a playing card. Ummm ....