ARandomGod said:
Ahh... but looking at that further I'm struck by the fact that an adjacent but non-grappling opponent would have a 50% miss chance when attacking into said grapple.
Only for ranged attacks, I believe. Melee suffers no such penalty attacking into a grapple (which is why having someone grapple while the rest of the party beats on the (dexterity bonus denied) opponent is a pretty good strategy). Also it is only a 50% chance when there are 2 grapplers. If there are 4 grapplers, then it would be 25% to hit each one, etc.
Again, I'm not entirely sure how this would work with the mage being grappled and the dragon not considered grappled (that is a difficult thing to adjudicate). Probably no penalty for the dragon in melee (except the cover I'd give the mage as stated earlier) but a 50/50 from other ranged attacks (from dragon allies) as the mage is half in-half out of the dragon's maw. I wouldn't penalize ranged attacks from party members trying to hit the dragon as the mage isn't any significant cover for it, obviously.
Also, the minimum CR for a Gargantuan chromatic dragon is 19 (a white) - so this was a challenging encounter, especially since dragons are known to be a bit on the heavy side if DM'd properly (and it sounds like your DM wasn't going easy by a long way
![Devious :] :]](http://www.enworld.org/forum/images/smilies/devious.png)
) . I'm assuming that you are at least level 18-20, so you should have some decent tricks up your sleeve to use against something like this. You stated that it was something of a surprise (not sure how your DM managed that one, but I applaud his resourcefulness

) though. Usually PCs are very aware they are going up against a dragon - its not usually sprung on them as a surprise, but I'm not privy to the details of the encounter.
In any case, it looks like you fared ok overall, which is good. (as long as you got your revenge on the scaly beast).