It is difficult to give good advice, as different players like different strategies. I believe you need a balanced deck as you need to win quests to get more cards and you never know what the quest rules will be. You need to remember that neutral cards can be used with other cards, thus if you have good neutral cards, they can help you in almost all quests. (The only time they cannot be used is for the rule set that state no neutral monsters)

I hope this helps.

!BEER