Looking at an ancient night twist vs. 15th level druid, my best bet is that your player neglected the defensive aspect of druid melee.
Many of the very good offensive animal forms suffer from low-ish AC, which means they are pretty vulnerable to damage in the long run. However, the druid has more than enough means to get a respectable Armor Class.
1. Wear armor with wilding clasps on it (or beasthide/wild enhancement). A Dragonhide breastplate with magic vestment from a friendly caster should provide 5(base)+3 enhancement=8 AC(at least)
2. Defensive spells: Halo of Sand (druidic version of shield of faith) gives up to +4 ac, bite of the werebear also provides a healthy amount of natural armor bonus(+7).
So, with these buffs, your Ac would have been 10(base)-1(size)+1(dex)+14(natural)+8(armor)+4 (deflection)=36. Not quite autohit (ancient night twist attacks at +33), but still not impressive. You can get much higher with other wild shape forms though.
The main strength of the druid is it's versatility. Unlike a traditional beatstick, you don't have to play beatstick unless it suits you at that particular time. As you could see, going dire bear vs. that thing isn't a very bright idea. What I would have done was either wild shape into a flying form (legendary eagle or desmodu/dire bat for example) and simply start raining spells and/or summons on it from range. alternatively, if i really had to tank it I would have gone with an ultra defensive form (cave ankylosaurus IIRC has somewhere around 20 natural armor and there are other even better at higher level). and focus on getting hit as little as possible, and still flooding it with summons and/or spells.