I am assuming you are referring to one of the top mounting bolts that holds the trans to the engine?

These are usually the most difficult to break loose because of the need for extensions to reach them and still have clearance to turn . Extensions greatly diminish the rotational torque than you can apply and the use of 4 extensions linked together diminishes it by quadruple, it is much better to have a single 2ft extension rather than 4 6 inch extensions linked together .
Sometimes it is much easier to take the hood of the vehicle off and use a breaker with just a socket on it from the top to get it loose.
Optionally you can put some of the other bolts back in and snug them up then lower the tail stock of the trans down as far as you can so you can reach up with an impact driver and loosen it.

 

No I was talking about the 4 bolts that hold the transmission to the bell housing