NOTE: Must be done in one level!
This achievement can be easily obtained on Day 32. The trick is to use to execute the chainsaw execution move on the zombies. Now, this can prove to be a bit tricky. You will want to do a chainsaw execution on one or two zombies, then run away so the others follow you, then get a few seperated from the group, then rinse and repeat until your multiplier is at 100x.
Alternate Method #1: Once you reach Day 14, as the level only allows you to use a chainsaw, simply keep using for the chainsaw execution to build your multiplier. Watch out for Big Guys, as you can't break their grapple.
Alternate Method #2: This can be obtained in the first Day of the Hell mode, as a few members have said they have had up and around a 200x+ multiplier.