Sniper's Huntsman, Fortified Compound or Medic's Crusader’s Crossbow required. The above weapons require "finding" via a Drop, Craft or opening crates. The Huntsman is obtained after 5 other Sniper pack Achievements.
The hardest is finding target that you can hit three times, and have them not die.
With the Huntsman or Fortified Compound "Quick Shotting" will deal at least 50 Health per Body shot hit. A fully healed Demoman, Soldier and Heavy can survive three shots without healing between shots. Medics may be able to take 3 hit due to their passive healing but they have 150 Health, means you may need to space out your shots.
Crusader’s Crossbow can hit for as low as 35 Health, but this is only at Point blank range, not the ideal place for a Medic. The Slow rate of fire will hurt the ability to hit the target.