The other solution here is good but I found a better method because it's less monotonous repeating the stage 7 over and over for just 10-12k points, if you're planning to get 100% achievements. For this you need adquire the DLC.
According to the others achievements like S-Hard run, every character's run on easy difficulty (incluiding the DLC characters), the perfect stage (no hit damage) and the arcade hard run, you'll get around 4 millions and 4,5 millions points (or maybe a bit less, idk). In my case I got 4,6 millions while getting all achievements but this one.
At this point you will know the enemies pattern so I recommend to run with Shiva character and his bitz attack move (
) on Very Hard, Maniac and Maniac+ diffculty. Just spam his blitz attack move as you can. You don't even need finish all stages. Until Stage 3 or 4 every diffculty is fine.
For example, on Maniac+ on Stage 2 I got S with 125k points just spamming his blitz attack move. Others stages I got C-B-A ranks on Very Hard, Maniac and Maniac+ which means less score obviously. In one hour I got 400k points running on these difficulties.
Good luck, this game is very good but it's so grindly getting all achievements.