With alot of aeg's there's a freaking lock washer between the flashhider and the outer barrel, I would try to saw that thing off.
If that doesn't work... try vice grips and a hammer/metal tube/clamp.
Mount outer barrel in clamp, (don't squeeze too tight)
Lock the vice grips onto the flashhider, tightly.
Get a hollow metal tube or something of the sort, use it as an extension of the vice grip handles, so you get some more leverage.
If that doesn't work, encourage more leverage by hitting the end of the metal tube with a hammer.
If THAT doesn't work, a 12lb sledge hammer to the flashhider will fix it...
Oh yea, and remember to turn
clockwise to loosen,
counter-clockwise to tighten, the threads are backwards on most guns.