I think everyone covered the various targeting methods. I should note that you can customize the party list order such that tanks are always on top (or bottom, or wherever you want them). I play PC with a controller so up/ down works nicely for going through the party list. I imagine setting tab to only go through party members (and shift tab to reverse) would be more or less the same experience. You can keep whichever target you want selected by hitting whatever your 'confirm' button is (X on PS3/4 style controller) after selecting them and before casting a spell.