This solution explains TWO ways to get this achievement.
I didn't see this up here, and I hate to be THAT guy that encourages glitching of a game, but here it goes.
You can duplicate weapons and sell them to vendors to get money to buy med-kits. Aim the weapon of choice at the ground, throw it, and then automatically hold the Y button. This should both throw the weapon AND drop it at the same time, creating two of the same weapons. Now hold X to automatically equip ONE of the weapons and rinse and repeat until you are satisfied.
Now go and buy medium medkits from a vendor after selling your ill-gotten gains. Then fast travel to the Monastery and head outside. Take the gate directly in front of you and follow the path out of the courtyard to this flaming car. Kill all of the zombies and Stand in the fire and just repeatedly use your medkits. I got 70 used in less than 3 minutes, 8 minutes if you count the time it took to get the money, buy them and travel to the fire to use them.
ALTERNATIVELY, if you've beaten the game and don't believe in glitching the game, you can start a new game with the character you beat it with, load up chapter 18 and do the final fight again. In a room in the back, there are a bunch of medkits to pick up. Just run outside and get shot and use the medkits, reload the checkpoint and rinse and repeat. This can take up to 25 minutes, depending on how many medkit uses you had prior, and is mostly due to load times. Refer to the YouTube video for visuals on this method, and credit goes to SNiPERxXV1 from YouTube for the video. (PS. Sorry if the music is too loud...)
Hope this helps, and if you vote negative, PLEASE comment and tell me why. Anything that helps you guys helps me as well. <3