THRESHOLD SYSTEM
Instead of using the average or maximum hp, list each monster's minimum to maximum hp range. Instead of subtracting hp, add up a total of damage. If the damage exceeds the minimum of the hp range, the monster enters the bloodied/threshold state. While in the threshold state, a monster can take a Threshold Action. Only one threshold action can be used at a time, and only at the end of another creature's turn. Spent threshold actions are regained after a long rest.

Most monsters only have one threshold action, Multiattack.

Additionally, while in the bloodied/threshold state, a creature is knocked unconscious or enters the dying state if its damage exceeds the maximum of the hp range or if it takes a critical hit while in the bloodied/threshold state.

For example, Gregor the paladin deals 24 damage to a wight. This causes the wight to enter the threshold state. The wight immediately uses its threshold action after Gregor's turn, attacking him twice with its longsword. The wight then takes its turn, attacking him again with his longsword and then using a life drain attack. On Gregor's turn, he attacks the wight again, scoring a critical hit with his greatsword that destroys the wight in glorious vengeance.
Gregor has dealt 60 damage to another wight, but has not yet scored a critical hit against it, and so it is enabled to use its Threshold action before it takes its turn as well, multiattacking twice against the paladin while his remaining two allies multiattack once, only having taken 12 and 10 damage respectively.

RANGE	DMG	THRESHOLD
24-66	24	YES
24-66	12	NO
24-66	10	NO
24-66	60	YES

BOSS MONSTERS
Boss monsters either use the Paragon system or the Legendary system and have maximum hp. Boss monsters include faction leaders and powerful enemies, like liches, master wizards, dragons, etc.