Continuing the advancement of the efreeti in this question, I try to figure out the gains of an efreeti advanced to 16 HD. After advancing it and achieving the huge size as indicated by its new HD, I want it to use its enlarge ability to reach the gargantuan size. So I figured out that in gargantuan form (after being huge because of its HD and after using its enlarge ability) the efreeti will now have the following stats:
+ 6 BAB
+7 natural armor
-3 AC/ATTACK (due to size changes)
Slam attack will go to 3d6 from 1d8 and
+2 feats due to HD.
Now my question is if I got the stats adjustment right, or need any corrections. As a bonus question, please propose a way to prevent this gargantuan outsider being one-hit KOd by a frozen attack (see frozen vulnerability). Magic items or spells are allowed, as this is going to be a boss and has minions, including spellcasters.
Edit (more info). The reason I'm trying to advance this monster is to create a challenging boss for this kind of party: levels 11-12. Wizard - blaster, Cleric buffer - support, Barbarian damage inflicter and a new rogue/ranger which a player will be playing for the first time, as his old rogue character met a horrible death. They have been able to fight monsters and characters at average party level + 2-3 until now without much of a sweat. The only times they have been close to death is when the wizard is disabled in someway and they are denied his massive damage/round.