This weeks challenge requires you to find a Heavy Sniper Rifle and do 200 damage. Just like the Bolt-Action Sniper Rifle from stage 2, the Heavy Sniper is extremely rare. The odds of finding one in a chest or as floor loot are close to zero, but there are still very reliable methods to get the Heavy Sniper Rifle.
For one, you can check vending machines around the map. The heavy sniper comes in and epic and legendary rarity. So keep an eye out for purple and orange machines.
The best way though is to open supply drops in a 50s playlist. As of this video the drops have a 15% chance to contain the heavy sniper rifle. So if you focus on supply drops you should be able to get 2-3 a match and find the rifle.