The maximum score multiplier is 5x.
To add to the multiplier and maintain it, you need to score gold on a section. This can be Shadow, Assassin or Silencer but it must be a Gold. Obtaining a gold rating usually means not being spotted or alerting enemies as you go through a section.
This means you must complete 5 sections in a row with a gold rating to achieve 5x multiplier and then a further two sections to unlock the achievement.
I couldn't really identify any particular sections that would be "easy" but I managed to get this in memory one. Memory 6 is also a good one to try this on as guards will ignore dead bodies and you will have access to all gadgets and accessories.