A Reaper award is given for getting 3 kills in 3 shots. Missing an enemy or scoring only a crit will reset the counter.
Your best bet for this achievement is to use a tank that has a howitzer, or 'derp gun'. This includes tanks such as the tier 4 Hetzer, the tier 5 M4 Sherman and the tier 6 KV-2. Tanks that have howitzer upgrades as an option can one shot anything a tier below themselves and some things that are within the same tier as well. You need to make sure to load HE when using a howitzer; the game will default to AP rounds on tanks that can use AP in their howitzers and this is not where those types of guns shine.
Also, this achievement is retroactive, so if you had any Reaper awards before this update, you will get the achievement automatically.