Go to the spawn menu, click the entities bar, select bouncy balls. When they spawn multiple things can be done with them but one of their more subtle actions is that one can press Use to eat the balls to restore 5 health or gets 5 more max health if the health is full. The achievement, specifically, requires one to eat 200 of these. If you look at the ground and hold E as you spawn the balls you can get the achievement really fast. Or, spawning a big group of them also works.