Set the following three keybinds to something close to home position:
Set Focus Target (I recommend shF)
Target Current Focus Target (I recommend F)
Target the Target of your Current Target (I recommend T)
These are actions that need to be used quite often, so my recommendations stand even if you already have things bound to those keys that you think should be bound to those keys.
When you get to a boss, set it as your focus target. Whenever you need to switch back to it, don't tab. Just target focus target.
If for some reason you can't target an enemy but you can target a friend, quickly target the friend then target of target.