give each persistent enemy a unique name, and give the GameInstance an array of names of killed enemies. when you spawn an enemy, check if you can find the name in the GameInstance array, and if so, don’t spawn the enemy.
if you want these enemies to stay persistently dead between game sessions, you can save that array to your save game. you can also use this same list for other persistent objects, like treasure chests or pickups that shouldn’t respawn.