The questions you bring up are good ones.

> > Hey Rawkhawk, I've got a question. How come using your FireUtil makes you weaker in cold areas, but also makes you immune to ice?
>
> How can you be weak to cold, but immune to ice? Ice is pretty cold.

First off, weak to cold but immune to Ice is in reference to direct attacks. Cold areas, until heated, will reduce the power of his attacks. However, any Ice-based attacks towards him won't affect him. 

> Also for water. Apparently the character can be evaporated but are immune to fire?

As for the evaporation point, it's a similar concept. Fire-based attacks won't cause damage, but any hot areas can cause any parts of his body that have been turned to water to evaporate. If he hasn't made any of his body like water, he'll be fine. 

> And how being made of earth makes you weak to ice.
Is this just Pokemon type rules as applied to the Monado?

As for the Pokemon question, similar, but not quite the same.