Cut Down should provide flat % armor or magic pen instead of a generic % damage increase

It has too much in common with Coup De Grace and just isn't worth taking over Coup De Grace in most circumstances. It would be a lot more interesting to double down on it's tank shred identity by giving it % armor/magic pen instead.

Something like gives 5% total armor or magic(adaptive) penetration against champions with 250 more maximum health increasing to 12% at 2000 more maximum health. The maximum health mechanic would of course prevent it from being abused by assassins unless of course they were using it for it's designed purpose(killing tanks).