From what I know, the only way up to the barracks is from crossing a bridge... which is on the other side of the base, opposite where you enter.
But basically, you can either slowly stealth through and take out the champions, or do what you did and go through a mad dash.
I can't explain it very well, but if you stick left when you enter the whole hort from the front, you'll eventually come to these boxes of some kind that you can climb/jump up on. Then you flip a U and fight everyone on your way to the barracks.
The best advise I can give is to sneak into the camp and stealth attack as many legions as possible. There is a seceret entrance to the right of the main gate (climb over the fence). From here take as many legions on the outskirts as possible (ones on their own or in small groups) before attacking the Champion.
When you fight the champion, use LT + Y to dodge his attacks as you will not get hit when he swings if you do this. Also, when fighing the Champion the other legions should stay back. After this head up to the barracks and again stealth kill as many as possible befor taking out the shaman. When fighting the shaman concentrate on the red poles and not the legions and you should be fine.
Finally, make sure you have health potions, axes and flame pots as they come in very handy.
Here's how I did it.