I think I understand your question is, Why is it different now?
Here are the most recent update patch notes regarding changes to followers. The Arena Champion was not changed, She now demands that the player character be involved in the fight. I have only found 2 thralls so far that were able to defeat her solo and even that I would say was by chance as they were very low on health at the end of the fight.
*Re-balanced followers HP and damage across the board.
- Reduced HP gain by an average 33%. Some high tier followers received an additional 16% reduction on their base values.
- Reduced melee damage by an average 40%.
- Reduced ranged damage by an average 30%.
- Removed the penalties for thralls who use attacks outside of their specialization.
- Reduced the HP for world bosses, dungeon bosses, and group content bosses by an average of 35-40%. *
Here is a link to the full patch notes should you wish to read them.