While I agree that a .22 mag is not ideal, it is what you have. When dealing with a small cal and large target your main concern is bullet penetration. Avoid hollow points and lighter bullets, find the heaviest solid bullet that you can and put sufficient rounds into the target to affect a change of attitude or focus.
This also applys to any cal. I like to think of it as a ratio of cal to mass of target.
As an example brown bear and 44 mag!
Never beat your head against the wall with out a helmet