Simple enough achievement, just play 20 games as Chaos, you don't have to win.
Some basic tips for fighting as Chaos:
Heretics may seem weak but their grenade launchers are great for taking out infantry who are attacking your Victory Points. Chaos Space Marines are a very versatile unit, but given Havoc's excel at ranged combat it's best to use the Marines as melee troops. Give them the tier 2 Mark of Khorne for this, and stick to using the Havoc's at ranged troops.
Beyond that try a few different units and see what works for you. You can try rarely using any units beyond T1, using T2 upgrades instead. But again you don't have to win the games so don't worry too much if you can't get the hang of playing as Chaos.