Since i dont know if modifiers are allowed for the achievements i would say the only level this is possible is the club. You need to beat all enemies in every room to be even able to reach this score.
A few tips:
- Taunting (pressing right on the D-pad) raises your multiplier by one, but it is very slow and should only be done from a distance to the enemies. I barely used it at all honestly.
Much easier to do is the focus attack Calbot which raises it by one as well. I would always have that ready in case you mess up.
- You dont really have to do fancy combos to get high points. Much more important is that you dont get hit to have a constant 3x multiplier. So i would always take it slow and be patient instead of rushing in and get surrounded.
- If you struggle with a specific enemy you should practice it in the hub area, until you can reliably dodge or deflect their attacks.
- If you get down to a 1x multiplier its not the end of the run. Keep calm and keep trying. Use calbot to raise the multiplier before killing any enemies.
- There is no point in waiting for a good moment to do a finisher on enemies, just hit it immediately before the chance goes away since it earns you more points than a regular kill. If they turn to a more powerful version its actually good, since you can earn even more points fighting them.
- The hardest fight is the room with the 2 miniboss fighters to the left of the dancefloor behind the doors with the keypad lock. I always went there when my multiplier was low anyway to not mess up my streak. When i had a 3x multiplier i went for all the other rooms instead.
In the end its a test of skill, patience and concentration. So the best way to do this is practice practice practice.
Good luck to you all!