Once done with the main story campaign you will have an option to skip it on every season start up.
You will need to create a new character when a new season starts.
To level that you generally run TOW tasks, Helltides and whatever color seasonal-tides, and Strongholds are a good way to get exp till 60.
Then you try to beat pit 20 to get to Torment 1. (or whatever # the new requirement is for torment 1)
As for difficulty level before Torment 1, it is irrelevant after the first or second time. Just pick one of the first two to get past it.
Necro can use shield, and they will probably introduce Paladin at some point.