As we know, she is apparently impossible to fix in terms of tweaking stats. So let's not touch those.
The main issue with Kalista is if you screw up her E Rend reset ability, you basically become useless till the cool-down is back.
Why not just add an indicator similar to champs like Pyke, Zed, etc. that shows you can get a kill + reset from your Rend with enough spears stacked into the enemy/enemies.
Of course things like heals can counter this indicator similar to the other champs listed above.
Basically don't change her stats, but give her an indicator to make Rend E resets easier for low-elo players. I feel this wouldn't affect pro-play since their timing is already very good with the champion. More clear E resets can definitely change Kalista's performance in lower tier. It's like the thing they did with Lee Sin's Q; an indicator would be nice please :)