I've moved this thread to RGSSx Script Support. Please be sure to post your threads in the correct forum next time. Thank you.
forcing_battler is not in the default scripts, and I can see no reason why a map saver script would introduce it. Also, Game_Interpreter lien 126 would NOT generate that error, unless you have modified that script or added another one with extra functions for the same class.
I suspect you have added OTHER scripts, and one of THOSE is the problem. Please list the other scripts you are using and include links to them as well.
"Trying other scripts" to overcome an existing problem is only going to make it worse.