Spear of Shojin's damage reduction is not a good mechanic.

Flat damage reduction isn't a good idea because in a lot of situations there are a lot more "hits" happening than you realize. In particular if a champion specializes on on-hits, with the way rageblade builds work, you have several things hitting on the same attack. Spear of Shojin reduces each one separately.

So for example, I was playing Trundle and having trouble in lane against a Jax. He rushed Rageblade and won a 1v2 against me and my jungler. Then I finished my Spear of Shojin and easily killed him 1v1, it wasn't even close. His damage was gutted by that passive. I mean, both his ult and W are reduced by it, while the underlying attack is also reduced. The Rageblade on-hit is reduced (twice I think because it's split physical and magic damage).

Another good example is if you are playing vs a DoT champion, yeah sure I know it's reduced by 70% against them, but these champs depend on Liandry's, and woops, that's separate from their abilities. So if Brand's passive is ticking for 2% and Liandry's is ticking for 2% and you have, say, 2000 hp that's 80 dps combined. If Spear of Shojin is giving you 30 DR (this is a lowball, I've easily reached 50 without hitting full build) then that's 9 DR. But it's 9 for each effect, so 18. So you take 62 in this example. That's ~23% less damage. If you also have Adaptive Helm, Brand may as well pack it up.

You have other edge cases like Syndra's ult which is 5 separate hits, so you get 5x the value of the Spear of Shojin passive.

I haven't tested Corki but I wouldn't be surprised if his autoattacks are double reduced too.

Basically, with the way damage works in this game, I don't think it's possible to balance this passive properly. There will always be edge cases where it performs abnormally well because of multihit abilities and onhits.