First off if you start off with /focus your going to change your focus to be your priest's current target. Also each seperate set of brackets before a spell is a different condition in which to activate the spell, thus combining your 2 conditionals is the way to go. Each conitional within a [ ] acts as "and" meaning all conditions have to be met. Also you don't need [exists] if you have [harm] as both [help] and [harm] implies something [exists]. You can also shorten /assist to be just /a and [mod] works just as well as [modifier] with less room taken.

So end result as written a lot shorter here will only cast if your focus's target exists, is harmful and not dead while your pressing alt.

/a focus
/cast [mod:alt,harm,nodead] Holy Fire