Plethora Sentry

local sensor = peripheral.wrap "left"
local laser = peripheral.wrap "right"

local function fire(entity)
    local x, y, z = entity.x, entity.y, entity.z
    local pitch = -math.atan2(y, math.sqrt(x * x + z * z))
    local yaw = math.atan2(-x, z)

    laser.fire(math.deg(yaw), math.deg(pitch), 5)
    sleep(0.2)
end

local mob_names = { "Creeper", "Zombie", "Skeleton", "Blaze"}
local mob_lookup = {}
for _, mob in pairs(mob_names) do
    mob_lookup[mob] = true
end

local function distance(entity)
    return math.sqrt(entity.x * entity.x + entity.y * entity.y + entity.z * entity.z)
end

while true do
    local mobs = sensor.sense()
    local nearest
    for _, mob in pairs(mobs) do
        if mob_lookup[mob.name] then
            mob.distance = distance(mob)
            if nearest == nil or mob.distance < nearest.distance then
                nearest = mob
            end
        end
    end
    if nearest then
        fire(entity)
    else
        sleep(0.5)
    end
end