I just took one apart last week. I didn't undo the one your talking about, I undid the one that goes to the fuel rail. I struggled with it for two days. Once I was 100% sure I had the tool fully seated into the fitting I gave my Gorrilla strength, and then just as i thought the whole fuel rail was going to bend pull out the fitting budged. I then was able to wiggle it out.
It appears the o-rings where what was hold it together so tightly. I just put it back togethether today and I had to use an excessive amount of force until I hear the "click".
Looking back now, I wish I would have applied some oil to the o-rings when I put it back together. Because, now I am thinkgin if Iever try to take it apart again it will bind up again and risk damage to the fuel rail to get them to separate.