I never succeeded the Kayn one. I'm just horrible at him, I'll admit that. But after spamming 4 to 5 games with him and feeding hard in all of them, I figured screw that, I'll probably need to play a ton of games to practice on him.
Maybe in my own time, since I don't feel like spamming a single champ over and over and over, heck I don't even spam Shen and Yi even though they are my usual go to champs. Why does it have to be a grind? Even with level grinding - my OCE smurf took 6 years to level. Yup, 6 years. I played 1 or 2 games every week or so, when the ping on NA was bad at my uni. But I didn't hate it, coz I could take my sweet time with it.
My suggestion: Make the quests really really hard, like the mastery 6 and 7 level ups for instance. But don't put a time limit on it so people can work on it at their own pace. Forcing a rush makes it unfun, and obviously if you make it available on bots, that would be boring too.