Being a technical deck user myself (well I just randomly picked something that isn't attack or defense-oriented), at first, I didn't bother to think which card/s should I focus on. But as I went on (specifically on Stage 3, where Miya can increase her ATK by 300 for a turn, provided that most of your cards are wood element), I took time to level the cards that has a skill that counters a specific element (e.g. choose 1 enemy character and attack. If the enemy character is Fire elemental, this card increases xxx ATK). Some cards that don't have any skill are viable if you think the ratio of their summoning cost and their ATK / DEF values are reasonable (can't give examples for this one, since people's opinions will vary for what is 'reasonable').
With same element bonus gains, I'm not even sure if there are any. And I used level 1 cards as fodder.
As for the experience cards that are given as rewards for beating certain stage bosses, it isn't a multiplier (I assume this is due to Aigis' exp cards just serving as multiplier). The exp cards given for beating the latter parts of Stage 2 and 3 can easily hit a card to Level 30 - 40 (NOTE: A single copy of that exp card is enough to max out a card's level). You get a notice anyway, something like 'you will hit the card's maximum level' in bold red letters.
I'll try to gather more info.