The problem with Dr. Strange is that he's a great crowd control character but is somewhat lacking in DPS.
When it comes to locking down opponents, confusing them or keeping them busy, BFR, assisting team mates, he's second to none.
But when it comes to actually taking out high-end opponents he doesn't exactly have that much firepower.
So he can win via BFR, either portal or mirror dimension, and he can definitely knock Venom around but outside of that I don't see how he actually knocks him out long enough to separate Brock. Maybe a fireball? Did Strange ever do a fireball?