I don't remember how I did it, it was some time ago, but my standard suggestion when failing an objective is: wait until you are stronger. The fact that I don't remember about it means it's easy when you are strong enough.

I haven't done tier 4, but I remember tier 3 being a nuisance. Try putting Karen in front to absorb some kills. If memory serves, pressing the left arrow key on area 1 will send you "back" to area 1, so if you need more gold but can't survive long enough, "reset" the area and get a few more kills before the horcruxes spawn. Do this until you've got some crusaders to outlast the area. I can't remember if horcruxes FULLY kill Karen or if they just suck one of her 9 lives. If the latter, have Mindy mimic Karen for that extra curse resistance. Splash damage will help keep the screen clear when horcruxes spawn, prioritize quest drops over spawn speed, and babysit the game to back out and reset an area before your back slot is nuked.

And pray for the best.