startup.lua

local modules = peripheral.find('manipulator')

local function fire(entity)
    print('Fired at '.. entity.name)
    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)

    modules.fire(math.deg(yaw), math.deg(pitch), 5)
    sleep(1)
end

local mobNames = {'Creeper','Zombie','Skeleton','PigZombie','Spider'}
local mobLookup = {}
for i = 1, #mobNames do
    mobLookup[mobNames[i]] = true
end

while true do
    local mobs = modules.sense()
    local candidates = {}
    for i = 1, #mobs do
   -- print(mobs[i].name)
        local mob = mobs[i]
        if mobLookup[mob.name] then
            candidates[#candidates + 1] = mob
        end
    end
    if #candidates > 0 then
        local mob = candidates[math.random(1, #candidates)]
        fire(mob)

    else
        sleep(1)
    end
end