The best way is to write it to the savefile.
In the saveGame script, add this at the end of the list of variables, but before file_bin_close(f);
file_bin_write_byte(f,global.trollkilled)
This will write the variable to your savefile when you hit a savepoint.
Now, assuming you're using the yuuutu engine or similar, you need to edit the saveExe script so that it loads the variable from the savefile when you restart.
Again, after everything but before file_bin_close(f); put this:
global.trollkilled = file_bin_read_byte(f);
Now it should save the variable.
oh, and a reminder to set the global variable in your warp collision event, instead of just the warp's variable. (global.trollkilled=true instead of trollkilled=true)