is she kept outside all the time? I heard maled dogs can "smell" a female in heat from MILES away. and will do anything to get to the female.
what have you done so far as training them together? I always see on those shows where they have 2 dogs out, walking far apart, then they progressivly get closer. Not that you will haveing them being dog friends but at least if you have enough good situations then they wont attack each other.
is it the St attacking or the boxer?
I was in a dead end situation a few months ago too, basically 2 female dogs, close in age (1.5 and 3) always fighting, it was getting to the point that blood was starting to get drawn.
Turns out it was me all along, when I finally stood back and examined the situation by not being emotional. I was the problem. Now I live in peace. I thought the alpha situations with the puppy where going to come back b.c we moved and they would try to be top dog of the new house. Nope , nothing!
LOL except 1 instance of girl humping girl last week and it was for only a split second LOL it could have totally been something eles. we have had no issue.
the biggest place of resistance in out old house was the yard, and living room. here I am boss, that is that. I own the whole house (in their eyes not my landlords LOL)
I was just to emotional about my "babies" . not anymore, they are still my babies, im just not afraid of offending them anymore.
sorry for that long post lol I dont even know if it helped you at all. I think you just need to figure out where the aggression is coming from