Here is the full list of all 20 Retro City Rampage DX achievements.
Avoid the cops at maximum threat level for 1 minute on foot, without taking damage
Kill 256 civilians within a single life
Beat Super Stomp Shenanigans in Turbo Mode
Kill four green sewer dwellers
Get gold medal scores in all sprees
Reach a 99X chain multiplier
Find all hidden loot bags, payphones and invisible walls
Knockdown three pedestrians with a single fling using the BioClaw
Collect all time machine parts
Hit 4 people with a single shotgun blast
Dethrone GLC's high score
Match all cards in a single round of the casino game at the Spades To Steal Casino
Play Free Roaming Mode as one of the guest stars
Hit 25 pedestrians in a row with your car without stopping
Knock someone off their bike to ricochet another off theirs
Get "sick" on "milk"
Drive 65,536 pixels
Blow up 8 vehicles in 8 seconds
Raise your threat level by committing a crime that attracts police attention
Beat Death Cam VHS without dying