1 of 2 ways (unless you want both ways.... if you do... i dont know what to say)
1. Travel to Fisherman's Refuge and complete Watchman Arad's series of quests, the 2 quest he gives you with a skill reward has it as its along with another skill depending on your secondary.
Or
2. Go to Droknar's Forge and Speak to Bartoch (Skills) and buy it there. Simple as that.
Lets say you have this stance up and you get hit by a Wild Blow. Wild Blow is always a critical hit and can't be blocked or evaded and it knocks you out of your stance. Does Wild Blow knock you out of your stance THEN do the damage? Or would Balanced Stance negate the critical and then end?