Last Whisper should only apply arpen for autoattacks
Assassins are running rampant simply because not only can they do their role in killing the squishies, but penetration items allow them to melt tanks and bruisers as well. If we simply nerf arpen however, tanks become rampant as nothing can kill them including their natural counter the ADC. We need a way to rebuild the triangle of Tank>Assassin>ADC>Tank. Making the arpen apply on autoattacks emphasizes it as an ADC item while simultaneously emphasizing the role of ADC's to counter tanks while reducing an assassin's ability to buy just one item in order to disregard target selection.