I am a HUGE ClickerSolutions fan so I second the suggestion to do some serious reading on that website. There are articles that address aggression. Look at "Training Articles" and scroll down to "Problem Behavior".
What your trainer recommended sounds a bit like the NILIF program but to an extreme. NILIF stands for "Nothing In Life Is Free". It is not about establishing dominance, but rather about establishing yourself as the leader. While difficult to explain, there is a
big difference. YOu might want to consider this approach:
http://www.k9deb.com/nilif.htm
But, establishing yourself as the leader isn't enough. YOu have to also help your pup overcome her fear of strangers. And yes, more likely than not, it IS fear. You will find some help on ClickerSolutions, but honestly, I think trying to find another trainer - a
positive trainer - is a good idea.