As the the trophy says, you must deal a total of 20 damage to a player in a single turn. The 20 damage can be spread across any way you like, through the use of spells, abilities and/or creature(s). This can be easily obtained in the second campaign battle or a Custom Duel against AI using the Green "Teeth of the Predator" deck, just play lots of lands and creatures, Blanchwood Armor, Overrun, and Giant Growth to increase your attack also keep up some pressure by attacking periodically forcing him to block and sacrifice his creatures.
You will also automatically achieve this trophy for clearing Challenge 7 as this challenge requires you to do deal 26 damage in a single turn to win. See Puzzling Master for details.