Solo running

In the finished game (after the elite 4 and champion are implemented) will it be plausible to win with only one pokemon? If the e4 are supposed to have competitive teams, does that mean their teams will be made up of lv.100s?

They will probably be around level 100, but you can still be higher levelled, as in this game it goes up to level 120.

Afaik, the elite four will scale with your team’s levels, so the elite four could be a very big challenge on solo runs.