A creature possessing the type construct, the type undead, or a subtype like elemental is immune to the condition bleed. Is there a way to overcome such a creature's immunity to this condition? I have a 20 level warrior focused only in bleeding effects (not my choice), now I have to level up and need to do something to not be useless against such creatures.
No. And that's for the best since bleeding ghosts doesn't make any sense.
The problem here seems to be that you are stuck with a build you didn't choose and that is apparently not adapted to the game your DM is running. You are trying to solve it the wrong way: just tell your DM you are stuck and look for solutions with him. Maybe he will ensure less foes will be immune to bleed, or give you something else to do in these situations, or even design a homebrewed feat for what you are looking for. Looking for petty exploits in obscure manuals makes no good.