no need to double stack

shoes of focus gives all the cdr kuku needs and more power than pen shoes
book of thoth is mandatory on kuku as it goes with his passive too well to pass up
poly is also mandatory as kuku has extreme power numbers meening your poly proc can likely hit for 600 to 800 plus whatever your basic attack does at that time
shard because it makes you hit objectives like a ten ton hammer as well as needed to make use of your power
gem of iso is utility generaly you want it but when you buy it will be different based on situation its very good in team fights as its kind of like having curse on a ten second cooldown
rod of tahuti because it makes you do all the hps of everybody

now we have all been in the spot were there are no enemies around us at level 5 if this is the case skip your ult and get more clear for freedom to get some juicy jungle xp your enemy wont know you don't have your ult I promise also if you level up after getting a kill with you ult and it allows you to point it don't as its wasted damage while on cooldown these small tips on when not to point your ult will be very very good knowledge as honestly you don't need it all the time and even when not leveled its scaling is insane at 120% magic power anyways

as far as basic level order and applications go your 1 does wonders level it whenever you can the slow scales and it make poly easy to proc and does loads on its own as well as sets you up for your 3 and 4

only level your 2 if you need it like if you rotate a lot and need to go back to lane but I strongly suggest only leveling when no other choice is available

3 is minion clear or follow up it can also be used to zone targets away or defensively if you run into your own 3 then an assasine would have to get hit by it and likely you 1 poly or you ult

your ult basically reads hide somewhere either behind friendlies or a wall and kill whatever it hits its also great when Fenrir tries to brutalize you pop ult and run into it RIP fenrir