As a THM I use a 360 controller and I tend to keep the main tank highlighted (but not locked), which allows you to use the up and down direction on the D-Pad to scroll through other members of the party (in target mode 1- the default) and select them if they are standing to one side or behind where my conal AoE does not reach. To target mobs is a little more tricky, but I do this through just using left and right on the D-Pad (and I don't remember it being much different in FFXI apart from missing some text commands like <stnpc>).
I also find that using macros helps a huge deal. All of my healing macros are /aoe on and the spell with a selection of <t>, other offensive spells are /aoe off so that I can select them from the bar and turn on the AoE manually if required (this way avoiding any accidents). I hold down the left and right trigger buttons on the controller to bring up the macro rows. As I am usually the healer in party using macros is also more comfortable for me, as the main ones that I tend to use during fight (Sacrifice III, Cure II, Paralyna etc) are together on the same row and it is easy to switch between them.