Toughest elite 4 member?

[Bubble Ba'ath]

Bertha was pretty tough for me. whiscash and quagsire are killing machines. Water/ground will alway be a beast combo. And my only water-type for the rest of her pokemon was Empoleon which also had a weakness to them, so I had to primarily rely on Roserade

 

[Luthor]

I'd have to say Lucian. His team can hit hard and while there are dark type options in Sinnoh I didn't end up using any of them that regularly so I had to rely on non-dark moves. The limited Ghost options in sinnoh don't help much either.

 

[KyogreThunder]

Lucian, as Alakazam is very fast and has sky-high Special Attack, and because Bronzong is bulky and has only one weakness.