yeah..... you kind of ran into the reason why BM fails in arena.
Anything that can survive your BW wins.
(well, almost anything)
You basically want to drain and control them as much as you can.
Slow them down with frost traps, keep your scoprid and viper sting on them as much as possible.
(if you dont use a scorpid it's more or less a lost match already as no halfway decent pala is going to leave Viper on him)
Basically, you want to attack their mana bars, not their healthbars.
Because when they are out of mana, they cant do crap anymore.
However, to do that you need a whole lot of control.
And control isnt BM's strongest point. I would advise you to try out (and practise a lot bevcause it's insanely much harder to play right) the
11/41/9 build.
You need to think of a lot more things, but once you get used to it you'll have much more control over your opponent.