If you're just interested in getting the badges: Pump everything into Energy/Stamina, spend what favour points you earn in game via levelling on the Premium Equipment. Then fight people. Fighting (if you win) gives you the best stat to stamina ratio. I'm getting like ~15 XP per 1 Stam.
The score scaling seems a bit off in that you can very easily get Golden Goose early on and get stupid-high multipliers while there are very few planes whilst later on in the flight there's no boost to scoring (as far as I know) and it's more of a struggle to survive than a score chance.
There's a bug, unless it's intended in the game, where you can actually start typing words during the 5 second countdown at the start of levels. You're then able to 'queue' words and unlock chambers prior to the actual spiders spawning.
Darknessone: You're forgetting the fact Zina has an attack that hits everyone and an ability that lets her inflict a stun/bleed effect on her attacks.
Also, Eye of the Tiger hits a character immediately and can't be avoided 100% with a switchout (depending on which character it targets). Also, if it kills a character (which if they have a crippled bench it often does), Zina gets 30 more energy, making it 50 energy regained that turn. (And a 'true' energy cost of -30 for the attack)
Compare that to Touch of Doom which can be switched out of 100% of the time and takes many turns to do its work and Touch of Doom and Zina aren't that bad.
Just an idea, to compensate for the fact that like 5v5 matches take longer than 3v3 matches. As an incentive to get people to play them why not give them a 5% chance or even a 6% chance to win a card from winning?
If people are still hunting for cards, they essentially have no reason to play 5v5 since its time-inefficient.
Try hitting the Challenges too for some decent experience! :)