WASDuk's Chris Patton outlines what players need to do to unlock the Red Son Deathstroke card in the latest Challenge Stage for Injustice: Gods Among Us on Android devices:
"The latest Challenge Stage for the Android version of Injustice: Gods Among Us offers a new addition to the Red Son line of card-fighters. The prize for fighting through the Challenge is a Gold level Deathstroke card. The card’s special ability increases attack speed by 5% for every Red Son card it is teamed with. Thus, if one teams him with Red Son Superman and Red Son Wonder Woman, then that team’s attack speed will increase to 15% (5% x 3 Red Son cards = 15% attack speed).
Similar to the previous Challenge Stages, players will need specific cards. Without these cards, players will not be able to enter the Challenge Stages, and are forced to credit grind until the cards can be purchased."