Err… Nerf everything?
Being difficult is subjective There are people who can solo cata FOW, there are people struggling to make the hop from Veteran to Champion… I remember the WoM Beta where as a Foot Knight I did 100k damage as FS ried the extremes of what was possible to change “difficulty”.
My honest opinion is that to make VT2 more difficult, there needs to be a wider breadth of type of enemy. The specials types based upon L4D2 are aging quite badly now. Have some enemy behavior that is truly different is the only innovative path. Chaos warriors, although tough to beat in numbers aren’t complex enemies at all.