In this challenge, you are up against a team of soldiers of different classes, but they can't see you as you have improved Shadow Blend, which unlike the normal Shadow Blend
Compare the normal Shadow Blend (this one) with the one in the Challenge.
, you can stay invisible forever even while standing and moving but still appear after shooting or being attacked. Beating this challenge requires you to use a powerful weapon, like a sniper rifle, homing explosive or shotgun; go for targets that are weak, like Assassins; and go for targets that are alone.
Also, avoid using an automatic weapon, since it takes a bit longer to destroy some targets and this gives a chance for the enemy to attack you. Either use Assassin with AWP to one-shot lonely enemy Assassins with Katana or try to kill them with the Omar shotgun. (Make sure no one is around, especially Commandos with the Commando rocket launcher as they have 300 DPS)