Homebrew Monster: Optimizod

[Frogreaver]

Let's design a monster with an ability like this.

Counter Charge (1 use per day): The Optimzod assumes a defensive looking position and starts to glow. As it takes damage it begins to glow more and more. This ability lasts 3 turns. The Optimizod can end the Counter Charge early if it chooses. While this ability is active the Optimizod cannot drop below 1 hp. When the ability ends the Optimizod shoots large beams out of it's sword at each enemy in range. Each enemy in range makes a dex save taking the damage they dealt to the Optimizod on a failed save or half as much damage on a successful save.

What base stats should I give it? Should the ability work any differently?

[Unoriginal]

Let's design a monster with an ability like this.

Counter Charge (1 use per day): The Optimzod assumes a defensive looking position and starts to glow. As it takes damage it begins to glow more and more. This ability lasts 3 turns. The Optimizod can end the Counter Charge early if it chooses. While this ability is active the Optimizod cannot drop below 1 hp. When the ability ends the Optimizod shoots large beams out of it's sword at each enemy in range. Each enemy in range makes a dex save taking the damage they dealt to the Optimizod on a failed save or half as much damage on a successful save.

What base stats should I give it? Should the ability work any differently?

I would just give that ability to several existing different monster statblocks which are working in the same organization.

[JackPhoenix]

How many players track how much damage they did over the last 3 rounds? Or is the GM supposed to keep track how much everyone did over that time?