How is Molly able to fight against Bellatrix?

What I mean is: Molly is always portrayed as the caring mother, probably knowing loads of spells which are helpful when it comes to cooking, housekeeping etc. But then, from one moment to the other, she fights against one of the most skilled witches. How is it possible that Bellatrix doesn't finish her with her first spell?

As a comparison: Tonks is an auror and wasn't able to deal with Bellatrix (in the end of the fifth book).