This brings up an interesting tactic for the Wizard or Elf:
Wizard first. Wait to the end of a successfully completed Quest and cast Rock Skin just before you exit the stairway. Next Quest, immediately cast Rock Skin again for two combined bonus defend dice. Now he's up to 4 defend dice.
Take it further. Avoid unecessary combat and treasure pulls, only casting spells at a safe distance or attacking from a protected diagonal. He pretty much does that already. Next Quest he has five defend dice. Rinse, lather, and repeat until the Wizard is enough of a brick to safely occupy monsters in a fight. By Quest 10, he's got 12 defend dice and is nigh-invulnerable! A wuss no longer, he can confidently Search with 9 defend dice for all the treasure he wants by Quest 7.
Now for the Elf. Same stratagy as the Wizard, but the attack power of your weapon makes you more than just a rock tank. First buy the helmet or the Battle Axe, then later the chain armor. Either he has 8 defend dice (with a helmet) and a 2 attack-dice shortsword after Quest 5, or he has 7 defend dice (with no armor) and a 4 attack-dice battle axe after Quest 8 (or after Quest 7 with 14 gold coins donated from each of the other Heroes).
The Barbarian and the Dwarf with their pool of Body Points will probably have their hands full protecting the Rock Skin Hero from battle before he is ripe (*Pop*

All gone! Or not -see my third reply down for an alternative -edit). Either would make a poor choice for the multiple-Rock-Skin spell effect, as their greater Body Points would be wasted.
Zargon would really be tasked by such an inflated uber-Hero, so he'd have to eliminate the threat early before the game swung irrevocably out of balance. On top of all that, if Zargon allowed the Heroes to regain spells after an unfinished Quest (as most groups do), then what's to stop the Heroes from only risking a minimum of rooms before casting Rock Skin, and then exiting the stairway? They then could immediately return for another Rock Skin boost. That makes it possible to cheeze a bunch of Rock Skin spells in a number of unfinished Quest 1's before attempting to successfully complete it. Hell, the whole group could have 12 defend dice when they finally decided to take out that pesky gargoyle Verag! Not that all of them would need to, as a single Hero could do it alone. So much for Mentor's advice about working together.
That nightmare could be avoided by playing to the letter of the rules found under p.23:
What Happens
Between Quests? As a Hero, your starting Body and Mind
Points are automatically resored once
you have successfully completed the
Quest and have returned safely to the
stairway. All spells are then returned to
the Wizard and the Elf.Note
once you have successfully completed the Quest. The group's revolving-stairway cheeze is halted, as a new Rock Skin spell wouldn't be available until the Quest was
successfully completed. To be successful, all the Heroes are probably needed, so Zargon would at least have a fighting chance to counter the Rock Skin gambit. Also note, however, that an already-cast Rock Skin spell could carry over into an unfinished Quest.
I'm betting most groups don't want this mess, nor the above rule to correct it. It's probably more trouble than it's worth. Therefore, I'll close my wall of words by saying Rock Skin should only be allowed to carry over if Zargon wanted to face the Rock Skin gambit and
A) possibly modify unfinished Quests to punish cheeze or
B) he used the successfully-completed Quest rule. In my opinion,
A could require too much adjustment, and for most
B is too restrictive (and also requires it's own modifications, see p.23,
Unfinished Quests). For the vast majority, it's probably wiser to simply rule that
Rock Skin doesn't last past one successfully-completed or unfinished Quest.
Personally, I'll try out Rock Skin lasting between unfinished Quests, governed by the successfully-completed Quest rule. As anyone can deduce, I have a passion for the HQ rules

and am intrigued by the rules interaction as literally interpreted. The tactics involved should make play more interesting... or cause all hell to break loose- who knows?