Lorelei

Lorelei is an Ice-type specialist. She is the first member of the Kanto Elite Four. She is also known as Prima in the anime.

As Kanto Elite Four Member
It is recommended that you use Steel or Fighting types because all the other types that have an advantage on ice types are weak from water type attacks, and vice versa. A dark type should help defeat the powerful Slowbro. Do not bring water types, because Slowbro, Cloyster, Jynx, and Lapras all have attacks that would affect water types, whereas your water types might not have a good move against them. Lapras especially has Thunderbolt, eliminating all water types. Magneton would be great for this trainer, as he is a Steel/Electric type, and Ice type attacks are not very effective against steel types. Also, Steel type moves like Flash Cannon are super effective against Ice types and Electric type moves are super effective against water types, which is what most of these pokemon are. On top of that, Psychic type moves of Jynx and Slowbro are not very effective against Steel types, and poison type moves like poison jab from Cloyster have no effect at all. However, water type moves will have normal effect on electric and steel types, so I recommend taking out Dewgong first and then sending in Magneton. Magnezone would be amazing for this trainer, made for it. If you get your Magnemite/Magneton/Magnezone to the lvl 50's range or get and have another back up type like a normal type with dark type or fire/rock type moves that is also in the lvl 50's range, you should be fine dealing with this Elite Four member. If you cannot, train all your pokemon to lvl 60, and then it will be easy.