Players can only do damage to enemies that are in close proximity to them.
Hero selection: All
Roadhog is absolutely needed here to make dealing with the snipers easier, and while you have to be close to do damage it's actually easier done than said. And no I didn't mess up that saying, as long as any one person is near an enemy everyone can do damage to him no matter the distance, making rushing characters VERY good for this.
Wreckingball, Roadhog, Reaper, Tracer, Moira, Doomfist, Junkrat, any strong CQ character will work wonders. Avoid snipers or medium to long range characters as you'll be useless to your team for the most part.
The strategy here is simple: Rush. Rush everything. Including the enemy tanks. With Roadhog and Wreckingball it's as simple as a Moira healing them while they destroy whatever it is they're shooting at and a Reaper is just the icing on the cake. Snipers are going to be the only issue you come across and as long as Roadhog hooks them then nothing in the mission will stop you getting the easy 15g.
If you would like help for this challenge and don't have anyone to do this with message me on Xbox and me and some discord friends will happily take you through.
My teams setup:
Wreckingball, Moira, Roadhog, Reaper.