Just get a gallon of gas and a match....it will be all over with before you know it. Okay, skip the gas and match thing. :lol:
I cut a lot of wood every year and I too occasionally come up with a jackpot like the one you describe. What I will usually do is climb my arse up the tree it's against, hook a 3/8" cable to about mid-point, but not ABOVE where you think it's reckless or the law of physics could come into play. I then take that cable and hook it to a vehicle or portable winch and run the cable backwards away from the way the tree would fall naturally. I then put some serious torque on the cable, but not too much, but enough. With enough practice you will know where to stop. After that I will give two face cuts on both sides of the tree thats not been cut yet. Give it a rest and see which way the groan is wanting to go. Possibly even choke the tension up a little on the cable to see if she wants to fall. Still not working, then I give it some hell on the backside of the tree, which would be on the backside of the direction of the cable being pulled. Cut er slow though, just a pinch at a time as depending how much tension you have on the line it may want to barber on ya. Just make sure your standing 90 degrees to how the cable is being pulled. What I've found is the trees will want to canter off one side or the other when doing back cuts, so watch yourself and if possible have someone stand back and see what the tree wants to do.
If none of that works or you're not comfortable with it, then skip it for another tree and let time fall it. Or, as I've done once, and only once, start cutting the fell tree into small pieces until manageable.