DJMAX RESPECT V

In the Settings menu there will be an option to enable the Freestyle Combo Chain. Once enabled, the combo in Freestyle Play will carry over between songs and will allow you to reach a combo higher than the number of notes in any individual song, which will be required for this achievement.

You do not necessarily need to hit 999,999 consecutive notes to get this achievement. By turning on Auto 5X Fever your combo can increase up to 5x more per note if you have a good streak going. If you happen to miss a note while playing a song, immediately backing out to the Music Select will cause you to lose all of your combo. Instead, if you miss a note, you should hit Restart, which will essentially reset your combo state back to how it was before you started that song, and then you can return to the Music Select if you don't want to try that song again.

Since they're both really grindy achievements, I'd recommend going for this at the same time as the achievement for getting 80% of the Master Score for any button. Just stick to the same button (4B, 5B, 6B or 8B) and go through as many songs on as many difficulties that you can get FCs on and eventually you should get both achivevements

Generally speaking getting to a 999,999 combo is going to take a fairly long time, and it means that you'll constantly need to go into songs with the intent on getting a Full Combo every single time. If you wanted to take a break from getting non-stop FCs and instead, for example, aim for S ranks on really difficult songs or something, you can go into the Settings and turn the Freestyle Combo Chain off, which will keep your currently chained combo saved and allow you to come back to the 999,999 combo grind any time you want.