I am currently making this build and I was curious as to why nobody has suggested maximize spell for heartbane? When occupying a spell slot 3 levels higher, the spell becomes a level 7 spell right? So creatures with lvl 4 spell immunity can still be affected by it, (I somehow recall some creatures being immune to it. Maybe it was a boss.) and the +5 DC effectively makes this equivalent to a lvl 12 spell (I realize there is no such thing as a lvl 12 spell). Is the DC high enough for that spell that increasing it with max is pointless? Thoughts?
Here is some more details on what exactly Maximize does, from NWNWiki:
Maximized spells apply all variable numeric effects — including damage, number of targets, and so on — at their maximum values.
"Variable" numeric effects means numbers that are not known before the spell is cast. That is, it means the result of dice rolls, not numbers based on the caster level.
The "opposed rolls" not affected by this feat (in addition to the already-mentioned dispel checks) include touch attack rolls, special attack rolls (e.g. Bigby's forceful hand), skill checks, spell resistance rolls, and opposed checks.
Rolls made only indirectly as a result of a spell — such as rolls resulting from a spell's effect but being directly caused by an attack — are not affected by this feat. This includes the chance for an attacker to miss due to concealment, damage inflicted by a polymorphed creature, bonus damage from an item property (e.g. darkfire), and damage inflicted by a damage shield (e.g. death armor).
One good use of metamagic feats is to effectively gain "emore" casts (so to speak) of a given spell by using it in a higher-level slot via applying metamagic feats. Even if there is little or no benefit from the actual application of the feat, this can be useful. For example with HB, even if Maximizing/Empowering/Extending (etc etc) doesn't really do anything for the spell itself, you can still get more flexible spellcasting that way.