Leopold's Applause and Elemental Weakness - Work Together?

With Leopold's Applause allowing you to penetrate hits down to -50% instead of the base zero, does this then allow it to stack with Elemental Weakness? Or if you are using Elemental Weakness and lowering resistances on monsters to more than -50%, does the penetrating hits not take effect / are factored into damage calculation?

It does work. It works with Rakiata's Flow too but instead buffs the monster because you're reducing the negative scaling. You simply hit with -%50 all the time but if you're using Rakiata's Flow you simply don't need ele weakness as it is in most scenarios going down to the -%50 without any additional penetration.

Related forum post:

https://www.pathofexile.com/forum/view-thread/3702703/page/1#p26425623