Stealth killer

Can someone tell me what the requirements are for the achievement / trophy “stealth killer”?

I’m playing on the Xbox one and I was never really tracking the achievement but now I want to I notice I’m 40% towards it already

I’ve stealth killed someone as they are walking, sat down and sleeping but my progress doesn’t improve anymore

Hoping someone can help

Kill 20 enemies by stealth.
So i guess bandits cumans etc

Not peasants.

Just realised that the 40% progress was from my other play line. Once I had more stealth kills on this playline it started to increase