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If it is the standard brake from SA, then you just need a good bench vise, (dont scratch the barrel, wrap it or use a barrel vise)
Use a good quality Armorers tool that has both 5.56 and .308 size cut outs.  The cheap ones just have the 5.56  size
Like a Wheeler  AR Combo tool
If you dont have one use a good wrench 
View attachment 40498
Edit:
I see your standard brake in your OP pic. 
Good armorers tool will have size to get it off