Can't Get EnoughG 20

The achievement progress bar (accessible by pausing and choosing statistics from the campaign) shows your progress out of 24. A free patch added more modes and maps that don't add to this progress.

Here are the only ones that actually count:

Modes:
Total Chaos
Escalation
Barrel Dash
Rampage

Maps:
Complex
Watch Tower
Vista
Factory
Cascade
Pipeline

Do each map listed on each mode listed, and you'll be good to go.

General tips:
Wrecking Crew is a swap-the-controller offline multiplayer mode. You can do it with only one player.

Set the number of rounds and time setting to the lowest possible.

Rampage is technically 3 minutes, but the time goes down as you use up ammo. Pick thermobaric rockets, and they'll shave off 30 seconds each time you fire them.