Uhm, do you have seen the table
? Because every single point you raised
is on this table. Big damage is +1; save or dies range from +2 to +3, depending on the specific type; spell levels are worth +1 for 1st and 2nd level, and +2 for anything above (which is bullcrap, but hey, its there); different kinds of immunities are listed separately and range from +1 to +2 and there is a generic "special defense" for +1.
Figuring out the XP value for any monster using those guidelines is actually pretty easy.