While nerfing Poppy, why don't you start with removing passive resistance bonus?

Pros:

• bonus stats are boring for both parties, and have no counterplay
• removing passive bonus stats makes situational stat bonus (<30% HP) feel more impactful, improving her Valiant Fighter theme
• same applies to Iron Ambasador
• 15% resistance bonus is more than it seems, especially coupled with pseudo-sustain of Iron Ambasador
• she needs nerfs

Cons:

• ?

I don't feel like removing half the damage from her ult if she uses it in combat (single-tap) is a better thing. As it grants your enemy a brief period of untargetability, cutting it's damage brings it close to "not worth casting" point, as you could deal the same, or more damage simply by hammer-smacking your enemy.