In boss mode we can fight enemies for 8 energy each, with the guarantee of either obtaining a whole card, or just a card puzzle piece, or both together. But getting a whole card is pretty unlikely.
We can also reset each phase, the costs are: free -> 8belly -> 18belly.
Info: After reaching level 114, the rest of the boss Mode (Breaking News + End of Era+Assemble! Set off again!) will automatically open.