I used your battle tips and finally completed Storm's cape. Thanks.
Happy to help =)
How do you know when to boost a card? I see that you can get card experience but I don't know how to gain that experience.
Boosting's kinda important. Boosting will raise the stats of a card. Usually quite a lot once maxxed, and a maxxed card will carry over more of its stats when fused.
You can boost any card, but it really isn't worth it for the lower ones. Anything below rare is a waste, imho. When you click on boost, there should be a card there - probably your leader - ready to be boosted. You can change this card if you wanted to boost some other card. Below that will be a list of your cards, sortable. Set it to display your cards by lowest rarity up. Check as many cards as you are willing to sacrifice (to the maximum of 10) to improve the card you are boosting, then click on the button at the bottom. Boosting gets expensive at higher levels, and there are rapidly diminishing returns. It's probably worth boosting any attack or defensive deck card you have a couple of times as you get enough feeders (cards) to do so.
Also, occasionally you can get lucky and get a 150% effective boost. Maybe 10% of the time.
I still don't understand what to do with the iso-8
ISO-8 defies the rules of boosting, and gives you 3 levels per card when used to boost, regardless of what level your boosted card is already. So it is best to use feeders for the first 20-30 levels of the boosting, then go to ISO-8 to finish off your cards, if you have enough. I'd say use ISO-8 only for SR or above. Your Captain America is prime for ISO-8, as it would get extremely expensive to max it with just feeders.
For the best end-result on a card, max both base forms levels, and max their mastery as well by using them in your attack deck successfully. Then fuse and max out again. Marvel uses 2 card evos (fusing). In Rage of Bahamut there are 4 forms of each card and up to 8 can be used for evolving, so it is quite a bit more time consuming and expensive to get cards to their most effective.