i would put item combo (wear or craft, mostly craft, as i have 99 spades, 100 armors...) .
if you mix for example ice ball + fire ball you get carbon ball which does more damage), or if you get red armor + dragon armor you get plus 20 more speed, and so, a little as minecraft.
Would be more fun, idealy same with books, as i have more than my local library...
You are right. I locked it at 4 when i noticed some high speed issues with more than 5 balls but i did not change the achievement :(