Okay so you roam around for a bit until the world map gets shattered/fizzles out/some animation happens.
Then you'll notice that on the battle screen there is a giant enemy called O-V-E-N. Now what you have to do is buff your party up with whichever spells you have at the time, it is an early fight so it depends on your party at the time.
Then you just do a lot of burst damage and on the side you'll notice that there is a little meter showing you how much heat the oven is at. Now if you relent in a round it'll actually go down, and if you take too long the O-V-E-N actually explodes so you have to pre-heat it before 15 turns pass, but it isn't too hard if you bought any upgrades for your party.
When it pre-heats it'll slump over and you are done.
And that's how you pre-heat an oven in dragon quest.
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Behold the majesty that is Doom. Feast upon his greatness, and know you will never measure up.
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