Stay to the sides and the lava won't hurt you. Killing a dragon entirely with traps will not give experience points. What i did was lay traps one at a time until I found out how many would kill him then i backed off one trap. This brings the dragon to something like nearly dead. If you then use your party to inflict the rest of the damage you get the exp points. It seemed to me anytime a monsters death blow was delivered by something other than a character, you got no points. I noticed this in that temple when the various monsters were killed by the gems on the pedestals.
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Balance