Yay i just got the second hard badge. I have a tip for people that are having trouble with. Win the game a few times so you have most of the cards. Then, to get your max mana up, do not attack any other castles. Wait for them to attack you. When you are in a battle, call your units to retreat every minute or so. This will give you more exp and more mana to fight the cave of trials.
This is the real guide; just reposting 1)Pick Virus. 2)Start in Canada, New Zealand, Cuba, Greenland, or Madagascar.If you do not get there restart. 3)Sell starting symptoms, if any, and buy sneezing (unless it is what you started with, just keep it). 4)Buy 1-1-1-0 resistances but no transmissions (they trigger closures). 5) Once three countries are infected, sell sneezing to get rid of visibility. 6)Wait until all countries are infected. If Madagascar closes its shipyards or is not infected in 30 days, restart. 7)Buy all four drug resistances, sneezing, coughing, and vomiting. 8)After a few days, unlock tiers 2-3, keeping whatever symptoms it gives you. 9)Buy fever, fatigue, diarrhea, pulmonary edema, and hypersensitivity. 10): Wait until everyone dies. Keep this comment alive. It really works.
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