Get 5 friends. Each of you guys choose Mei (ice girl). Only do this on defense. All of you guys rush to the enemy team's spawn while the timer is going down. There should be 3 exits. Put 2 people covering each exit. Right when their timer hits 0, build a wall on each exit. The second a wall breaks, have person #2 build another wall. By the time their wall falls, you should be able to build a wall to replace their broken wall. Rinse and repeat. Tried this with only 4 Iceclimbers and this strategy held them at their spawn for 45 seconds to a minute. After they broke out we just spammed ice walls at our objective and still won.