It is an interesting mechanic and I like the idea that you increase the drama by presenting a group of "henchmen" easy to kill, which are mostly one-shot murders.
If I were using this strategy, I would use the Commoner from the Monster Manual (p.345). They have a AC of 10 and alone 4 HP each. Can be of any humanoid race too. Then you could use this as your base. Since they only have 4 HP on average, it would rarely take more than one hit or attack to kill one. his skill scores are all 10, so they have a +0 to all their bonuses. In addition, they only deal (2) 1d4 maximum damage. Therefore, even a large number of them should not pose a great threat to an adventure party, especially if you have members of the group that already have area spells of effect.
I hope this helps.
1st level Sleep On average, the spell should be able to eliminate at least 5 commoners. (PHB p.276)
A spell like Ball of fire It would effectively eliminate any number of commoners from those caught in the area of effect because the damage is a minimum of 4 HP, even if they succeed in their save rolls and you rolled 1s on all dice. (PHB p.241)