Finally finished the challenge
The gyms didn't pose much of a challenge, mainly because they didn't had evolved forms themselves and the type matchups were always in my favor at the end (Grovyle against the water gym, Seadra's surf against Solrock/Lunatone).
It became a lot more difficult in the elite four and I failed a couple of times before I realised I had to do a LOT of grinding. This resulted in my pokemon being around lvl. 60 before I could finally beat them.
The Dark type member was defeated mainly by a combination of pokemon.
Pure darks were defeated by Focus punch, grass types by Fly and Ice beam and Sharpedo by Leaf blade.
The Ghost type member was quite easily defeated by Seadra using Surf.
The Ice type member was really hard, but by learning Vigoroth Thunderbolt and Focus punch destroyed her eventually.
The Dragon type member was the easiest by far, all OHKO'd by Seadra's Ice beam.
Steven was really hard... Skarmory was defeated by Shock wave, Cradily by psychic (eventually...), Aggron, Armaldo and Claydol were easy because of Seadra. Metagross was defeated by a combination of Surf attacks and Earthquakes.
This challenge took a lot more time than I had expected, mainly because I had underrated the power of evolved pokemon...
My team:
The proof: