heres a tip from a guy who has about 700 games with yorick
get 20 magic pen and 4 armor and 3.5 mr in yellow runes get deathire touch then you MUST get folowing core items:
tear- norm boot - black cleaver/sorcerer boots(if u have gold to finish sorc boots get them first then black cleaver) after that you just build DEF if they are magic heave with alittle AD always plate armor if they are M heavy with a very behind ADC spirit vis or banshee if they are AS heavy AD's randuins
get ignite
max E AND W dont put a point in ultimate
u can beat ANYONE with that build even RIVEN almost 90% of my gamesi get an S atleast regardless if we won or not because sometimes u get a shitty uncarryable team
most damage dealt with my genious build + alot of farming + not dying and getting kills .. ward alot etc
u most likely didnt get an S because u didnt help objectives alot didnt ward much and didnt get alot of cs (98) by 24 mins
.. i know u were focusing on kills but if u had 5 kills and like 180 cs you wouldve got an S