I'd like to know how a person can successfully get through the EA content with only one character. I have tried that, and on explorer mode, it feels impossible without boosts to my action/health pools.
By "EA content" you mean Early Alpha?
I've been through the whole DoS2 content on Classic with several different builds in almost every patch.
For general advice, I start by farming as much experience as I can, working through each encounter in ascending difficulty.
I maintain the best equipment I can throughout.
I plan my talents, skills, stats, race, etc. around getting the most power and synergy from the character as possible (within the scope of the build I'm trying at the time).
I try to start a fight in a position with high-ground or in a manner where enemies will have a hard time reaching me.
I often kite enemies or separate out a member away from the rest with Teleport.
Most of doing well with a solo character boils down to preparation.
If you haven't gone through the content much, saving and reloading on tough fights and trying different approaches can help a lot.
Do you want to know anything specific?
On another note,
Just because it is possible to use just 1 or 2 characters to beat the content isn't a reason for us not to have a talent like Lone Wolf. Many people prefer using fewer characters but don't wish to take a handicap. The Lone Wolf talent was fun because it provided a different style of play.