You get Wildcards by opening boosters, competing in events, and winning games to beat your daily and weekly challenges - so if you're short on Wildcards for the shiny new Arena deck you want to build, the best thing to do is get playing with the decks you already have
Make the best choice for your needs and join the community of players who have already succumbed to the call of Rise of the floodborn
It's also a good resource for figuring out how much your cards are worth once the new set is out, so if you pull any Surge Foil cards in your booster pack preorders, you might suddenly find yourself with quite the nest egg
The roll number determines how many cards are exiled and can get played that turn
In general, I think Asmodeus will sometimes feel like more downside than upside in Limited, but the amount of power he brings Is pretty amazing, though certainly not bomb level