I'm kind of confused by your dislike of Pokemon that can die in one hit.(...) I don't know what specific Pokemon you mean by that.
I meant pokémon that are somewhat stronger and at the same time more vulnerable than the average. For example there's Weavile with its 400% Fighting vulnerability and low defense to begin with.
Another pokémon that's vulnerable in itself and not because of element effectiveness is Haxorus. Back when I was playing Black for the first time I got one for my team and failed to find a good use for it since I had a faster pokémon that was only slightly weaker and had more defense. Most of the time Haxorus got beaten pretty quickly by a same level opponent around the end (I think it was the Getsis or N battle.)
You will need a Water-type though, for useful coverage since an Elite Four member is a Fire-type user and to make overworld traveling easier.
Hmm. I could beat Black and Emerald without one and trying to do the same here. The team I used in Black consisted of Serperior, Scolipede, Krookodile, Galvantula and then Haxorus. Only Serperior, Krookodile and Galvantula prooved useful. (Yes, the beginning was pretty hard because of the double Sewaddle before the forest.)
So anyway. What I'm getting at is I replace water with ground because it's more useful and has immunity. Torterra and Mamoswine are both bad against fire though, so I still need to figure out what to use.