This achievement is really self explaining and straight forward. After you completed the last stage of the game on Normal Mode, you unlock "Heroic Mode" which makes the enemies and obstacles deal way more damage than before. This makes the game way more tricky in certain areas.
Now, playing the new "Heroic Mode", just shoot and kill 500 enemies. Every single enemy counts but you will probably need to repeat a few levels to farm even more kills. Stage 7, which is the second red level, has the most enemies so I would recommend this stage to grind out the kills you need for the achievement. Just keep playing, it will take some time.
Again like I said in other solutions before make sure that you always complete the level and get the "saved" sign on your loading screen. This makes sure your kill-progess counts and you don't lose a lot of time.