CS is probably creep score and you call critical strike "crit"
So, these probably affected your rank in that game. KP is 36%(15/42). Probably needs to be higher. Would say above 50%, but can't be sure.
CS is 122/300(I'm using 100 cs per 10 minutes, this apply to top/mid/bot lane, but I don't know the actual point when the game gives it to you)
I'll say 55%-70% is near the starting point for maybe S or S- for CS section, but I don't know.
Warding might have also killed your rank, but I don't know if it was on you.
Not sure if taking the credit for the tower being destroyed will help. Same for inhibitors.
AD=attack damage
AP=ability power
MR=magic resist
Learn which champs are AD/AP, then there is times where you would want to build armor/magic resist to counter some dmg
xin Zhao is AD, ekko is AP, there are some who does both.
AS=attack speed
wave management is pretty hard to explain, but you might want to hold off on it until you get at least better at last-hitting minions for a better CS. You can try to look up videos on it where pros explain them. Plus people who play normal, most of them don't use it. But I won't explain it(cause I can't)
(But at the same time, wave management helps with your CS a bit.
You can try this, go to a custom game, which is different from bot and normal games,
1.lock your level and maybe buy a starting item(doran blade, doran's ring, doran's shield),
2.go to a lane where you play Xin Zhao, and
3.last hit minions with auto attacks(no abilities) from that lane up to the 10 minute mark and see how much you can get.
Also, you can't recall back for items and use the teleport(or anything that helps you get to lane quicker. like the revive. Hopefully, you don't find some loophole to give a higher CS or do something wrong if u want to do this) )
Last-hitting is hitting a minion once to kill the minion before it dies so you get the gold and +1 CS. Forgot how much cannons gives. it might be 1 or 2.
Another tip: You don't always have to kill the minions as fast as you can. But you still need to last hit it in order to get gold and the increase in CS. I won't explain this.
How often you check your map?