This is an "grind" achievement, you will get it over time when playing Engineer.
The fastest way is to build your Dispenser near the front lines, where other players can step back to heal and replenish Ammo. While you shouldn't build your Dispenser near another Dispenser, you may want to note where they are for future matches, and see how often they're used.
Many Attack/Defend and Payload maps will flow better having a good Dispenser placement for your team.