It's really helpful to /keybind the function "Target Nearest Enemy" (on the Targeting tab) to something very close to your hand's resting position, like G. Also the Target Nearest NPC or Object function, which I like to have on V. These two keybinds will make targeting things trivially easy.
As a healer who plays with a mouse, you might also like using mouseover macros for your healing actions. These will cast on any party list slot that your mouse is over without you having to change targets, so you can just always have the boss targeted. They will also work on the Target of Target healthbar. You can also mouseover a character model, but the party list is easier because the slots aren't moving around in the middle of combat.
Macro example:
Code:
/ac "Afflatus Solace" <mo>
/micon "Afflatus Solace"
The /ac line uses the action. The /micon line sets the macro icon. The <mo> is the mouseover target tag.