There are few options you could look at to make it easier for you. First, there is an "Implied Target" option (or something along those lines) that will highlight whatever your target has targetted. Might only work in groups but that's a minor issue.
Also, you can set your UI up so that health and power is always visible over the heads of mobs etc, even if you don't have them specifically targetted.
Admittedly, it's not as sleek as a proper target of target window, but it might help keeping up with fight progress.
