The Truth exotic quest has a one challenge that requires you to beat 3 Nightfall Strikes, but not only do you need to beat them, you need to actually beat them with a power handicap. The Handicap on the Map Fragment says that you need to have a Power Handicap of 100.
Changing the Power Handicap on Nightfall Strikes is actually done by accessing the Five of Swords Challenge Card in your inventory. In the Inventory you simply need to click on the details of the Five of Swords card and then you can adjust the Nightfall modifiers to what you want. In the case of the Truth exotic quest you’ll want to increase the handicap for power up to 100. This will make your effective power level 100 less than what it shows on your character screen for the purpose of the Nightfall Strikes.
Once you’ve completed this challenge you might want to move this back to zero, unless you’re tackling other challenges. You can use the increase and decrease icons in the Nightfall Modifiers section of this screen to either increase or decrease the handicap between 0-260. As you increase the handicap you will see that the Score Multiplier will also increase.
This can also help in the instance of a weekly challenge where you need to get a certain score level to receive powerful gear.