If you need to spam some medi-gel uses, the Pinnacle Station DLC does the job nicely. Every time you enter the simulation, you're given a full load of medi-gel for free (ironically, Admiral Ahern occasionally yells "That medi-gel ain't free!" when you get shot).
The Survival mode works best, as the enemies will attack you and give you plenty of opportunities to use your gel as long as you can stay alive. As soon as you run out of gel, die and restart the sim.
Other ideas for "boosting" your medi-gel use:
Run out into danger so you're more likely to get shot, and use gel early and often.
When you've cleared out an enemy hide-out, find any plasma containers or other explosive "barrels", walk right up to them, shoot, and immediately heal. Repeat as long as you have gel or barrels available.
You can always replenish your medi-gel for free from the Normandy's sick bay. You can return to the Normandy from a planet without losing your progress on that planet, so return as necessary to get more gel before going to the next building on the planet.
Obviously this achievement may make it difficult to get the Tactician, as you can't use medi-gel unless you are injured, and Tactician requires minimizing your injuries. It's possible, but hey, you have multiple playthroughs ahead of you in this game anyway, so it may be easier to not stress it and go for Tactician on one playthrough and encourage damage and medi-gel on another.