Rubber Bandits

On the last level in arcade mode the money is a diamond. Collect it and exit
You must kill all the guards for the door to open

4:00 Successful Escape
4:35 Striking Gold
5:25 Gone in 3 Rounds
5:25 Outclassed
5:55 Greed is Good
6:20 Anger Issues
7:00 Bye Bye Birdie
8:10 Mastermind
8:35 Oceans One
9:55 Diamond
12:35 Hail Mary
13:10 Why Thank You
13:30 Inhuman Reactions
14:05 Clean Sweep
14:35 Last Man Standing
15:30 Dead Weight
16:15 Super Cop

Achievement Walkthrough:

You find diamonds random either in heist or in arcade. Finish the level with the diamond.

Check out the video for a fast guide to all achievements:


Timestamps:

00:00 Intro + Successful Escape Achievement M
01:41 Out On Top, Gone in 3 Rounds, Outclassed, Getting Slick, Mastermind Achievements
02:59 Talk about multiplayer + Super Cop Achievement
04:05 Dead Weight Achievement
04:42 Hail Mary Achievement
05:08 Why Thank You
05:33 Inhuman Reactions Achievement
05:54 Clean Sweep Achievement
06:10 Last Man Standing Achievement
06:33 Oceans One Achievement
07:50 Bye Bye Birdie Achievement
08:30 Striking Gold, A Robbers Best Friend Achievement
09:01 Greed is Good, Anger Issues Achievements + Outro

This can be done in the "Last Heist" in arcade mode.