We really need a tank shred spirit item.
The thing is that Tankbuster is a conditional item (requires a burst ability) that some heroes apply much better than others, and that the damage doesn't scale with the ability damage/spirit power. Which means that a Geist that builds Ability Range and zero Spirit Power gets the same value as an 160 Spirit Power Grey Talon.
They need to add a percentage penetration spirit item (that's a multiplicative Spirit resist shred instead of an additive one).