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