I play using standard Blizzard UI coupled with WA's. Doing Mage Challenge Tower and I've found that default Blizzard enemy HP bars (mid-upper part of the right side) are too small. Any addons that increase their size without showing casts and other complicated stuff?
I would suggest pitbull or any of the other unit frame addons honestly. If you only want to modify boss frames then you can disable everything else in pitbull, its extremely customizable so you don't have to have castbars or any of the other stuff you don't want.