As trophy's description, you need to score 275+ runs in a 20 Overs match(Single Player). Choose Casual Match, select any teams, Overs: 20 Overs, set the difficulty to easy, then start the match. you need to win the toss and must choose to bat first in order to score 275+ runs or the opponent(AI) should opt to bowl first so that you can bat first. (If you didn't win the toss, quit to the main menu and start the new match again till you won the toss or getting bat first)
Since it's an easy difficulty match, you can hit all balls to SIX using
(try to hit down the ground, it's easier to score six for almost every type of deliveries) so you can score 275+ in less than 10 Overs (I scored 275+ in 7.4 Overs) After scoring 275+, you don't have to play the rest of the overs, just try to lose your wickets by yourself (play a defensive shot and start running, there will be high chances for run out) Once your last wicket falls or 20 Overs completed, Trophy should pop!
The Top That! trophy in Big Bash Boom worth 46 pointsScore 275 runs or more in an innings of a T20 in any single player match.