What difficulty are you playing. The devs did a pretty good job in my opinion of actually ramping up the difficulty at the higher settings.
Anyway, as for gargoyles.. They do have high elemental resists. However they have negative resists to spiritual and etheral damage. They resists to slash bash and poke aren't too good either.
Also, on tough enemies... if you don't mind a bit of cheese, give one of your characters a bow or crossbow. Have the other character hit the monster until he is the one being attacked, then just run around in circles (don't get close to the archer) until the monster is dead. He won't be able to hit you.
Another tactic that will gain you a few seconds anyway... even if you don't use your dolls for anything other than mules.. they can take a few hits for you. Pause the game, summon the doll, command him to attack and your other to retreat. Once the monster is attacking him, have your other characters join in the combat. He goes down fast without any points into him.. but in tough battles he can save you a potion or two.
As to the key and pit.. you will eventually return to the level. It doesn't really matter though since I think you can find the second key shortly after anyway (search the forum for their locations.. I don't have off hand, sorry). Once you have one of the higher level keys it automatically gives you access to the areas that the lower level key(s) gave.
Above all save often and in several different files.
