Quick Enemy AI Example

local Enemy = {}
Enemy.__index = Enemy
local Tag = "Enemy"

--//Services
local Players = game:GetService("Players")
local ReplicatedStorage = game:GetService("ReplicatedStorage")
local CollectionService = game:GetService("CollectionService")

--//Paths
local Modules = ReplicatedStorage:WaitForChild("Modules")
local Util = Modules:WaitForChild("Util")
local EnemySpawn = workspace:WaitForChild("CombatTesting"):WaitForChild("EnemySpawn")

--//Modules
local Janitor = require(Util:WaitForChild("Janitor"))
local HumanoidList = require(script.Parent.Parent.HumanoidList)

--//CONSTANTS
local AGGRORANGE = 50 --How many studs away player needs to be for enemy to attack
local ATTACK_DELAY = 2
local KILL_RANGE = 2.5
local ATTACK_DAMAGE = 20
--//Private Vars
local States = {
    "Searching";
    "Attack";
}
local AttackDebounce = false

local function GetDistance(p1, p2)
    return (p1.Position - p2.Position).magnitude
end

local function GetClosestVisibleTarget(enemy, targets, blacklist)
    --NOTE WE MAY NOT NEED BLACKLIST SO STOP COLLECTING IT
    local distances = {}
    for _, char in ipairs(targets) do
        if CollectionService:HasTag(char, Tag) then continue end
        distances[char] = GetDistance(enemy.PrimaryPart, char.PrimaryPart)
    end
    local low = math.huge
    local index
    for i, v in pairs(distances) do
        if v < low then
            low = v
            index = i
        end
    end
    return index
end

--//Enemy Functions/Methods
function Enemy.new(instance)
    local self = setmetatable({}, Enemy)
    self._janitor = Janitor.new()
    self.Instance = instance
    self.Humanoid = instance.Humanoid
    return self
end

function Enemy:Attack()
    if self.Instance == nil then warn("An instance is required in order for the enemy to attack.") return end
    if self.Target == nil then warn("A target is required in order for the enemy to attack.") end
    self.State = States[2]


    local function takeDMG()
        while true do
            self.Humanoid:MoveTo(self.Target.HumanoidRootPart.Position)
            self.Humanoid.MoveToFinished:Wait()
            local plr = Players:GetPlayerFromCharacter(self.Target)
            if not plr then return end
            local distance = (self.Instance.HumanoidRootPart.Position - self.Target.HumanoidRootPart.Position).magnitude
            if distance <= KILL_RANGE then
                local playerDefense = plr:GetAttribute("Defense")
                if playerDefense then
                    local hp = self.Target.Humanoid.Health
                    local amt = (playerDefense * hp)

                    self.Target.Humanoid:TakeDamage(amt)
                    AttackDebounce = true
                else
                    self.Target.Humanoid:TakeDamage(20)
                    AttackDebounce = true
                end
                task.wait(ATTACK_DELAY)
            end
        end
    end
    task.spawn(takeDMG)
    local distance = (self.Instance.HumanoidRootPart.Position - self.Target.HumanoidRootPart.Position).magnitude
    if distance > AGGRORANGE then
        self:PrimeForAttack()
    end
end

function Enemy:PrimeForAttack()
    local keepGoing = true
    if self.Instance then
        while keepGoing do
            -- Get list of all nearby enemies and non-enemy humanoids
            -- Enemy list is used to ignore other enemies during later detection
            local humanoids = HumanoidList:GetCurrent()
            local friendlies = {}
            local characters = {}
            for _, object in pairs(humanoids) do
                if object and object.Parent and object.Parent:FindFirstChild("HumanoidRootPart") and object.Health > 0 and object.WalkSpeed > 0 then
                    local torso = object.Parent:FindFirstChild("HumanoidRootPart")
                    if torso then
                        local plr = Players:GetPlayerFromCharacter(object.Parent)
                        if plr then
                            local distance = (self.Instance.HumanoidRootPart.Position - torso.Position).magnitude
                            if distance <= AGGRORANGE then
                                if CollectionService:HasTag(object, Tag) then
                                    table.insert(friendlies, object.Parent)
                                else
                                    table.insert(characters, object.Parent)
                                end
                            end
                        end
                    end
                end
            end

            local target = GetClosestVisibleTarget(self.Instance, characters, friendlies)
            if target then
                keepGoing = false
                self.Target = target
                self:Attack()
            else
                self.Humanoid:MoveTo(EnemySpawn.Position)
            end
            task.wait(3)
        end
    end
end

function Enemy:Init()
    print("Enemy was initialized")
    self.State = States[1]
    self:PrimeForAttack()
end

return Enemy