Batman is actually 1-1 against Deathstroke, that's better than 3 stalemates (one of which where Deathstroke was stated as toying with her).
Note, he said she was toying with her but she was visibly winning, she had a sword to his face. The toying with bit is the only thing that makes it a draw. If you want to be nitpicky enough to not call that a draw, I'll point out he actually ran from all three fights.
I'm pretty sure Batman's fought Slade more than twice... but hey, he jumped Slade and got worked. Slade fights Cass with no prep or ambush on her part and has to pull back. Slade even admits that her fighting is perfect and the only way he, *Deathstroke* can get to her is by psychology.
Cassandra he needs psychology, Batman he needs punching.