Firebrand (Spell Compedium) is incredible with dazing spell metamagic (and if you use searing spell as well so you make sure no one is fire immune.) I am summarizing the spell below
Wiz/Sorc 5, Medium Range (so 200ft range at CL 10), 1 5ft burst for every caster level (thus 20 bursts at 20 CL), reflex save for half
Each burst does 9d6 fire damage at CL 9 or 10d6 fire damage at CL 10.
In addition if a creature is hit when the spell goes off he receives 4d6 fire damage at CL 9 or 5d6 fire damage at CL 10 the following round in damage.
If a creature enters the area of a burst within 1 round of the spell going off he receives 4d6 fire damage at CL 9 or 5d6 fire damage at CL 10.
Material Cost 20gp
So combine with Daze Spell and you can pretty much hit every target you want to on the battlefield and be selective about the targets you do not want to hit. The targets that are hit are dazed for 5 rounds.
Yes this will be an 8th level slot, but this is practically a guaranteed combat ender. Or you can use a metamagic rod that will cost 54,000 gp on open market (available after level 16), 27,000 gp if crafted (available after level 11).
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Human Race, Level 13 build, wizard
Traits: None used
Feats
1 Spell Focus Evocation
Human 1 Greater Spell Focus Evocation
3 Elemental Focus Fire
5 Greater Elemental Focus Fire
Wizard 5 Searing Spell (metamagic feat from Sandstorm
7 Dazing Spell
9 Persistent Spell (pathfinder version)
Wizard 10 Heighten Spell
11 Prefered Spell Firebrand
13 Spell Perfection Firebrand
So at level 13 your Save DC is
10 base
+5 Spell Level
(you can cast 7th level spells, but I am assumming a normal 5th level slot)
+your ability modifier I am assumming a 26 int thus a+8
(17 int+2 human, +3 levels, +4 item which you will upgrade around level 14 to 15)
+1 spell focus
+1 greater spell focus
+1 elemental focus
+1 greater elemental focus
+4 spell perfection
=23+Int or a save dc of 31 assumming 26 int
So every monster with a reflex save of 11 or less will automatically fail unless they roll a natural 20. The average reflex monster of cr 11 is 10, cr 12 is 9, cr 13 is 10, cr 14 is 11, cr 15 is 12, etc
Lastily you don't need to prepare any firebrand per day. You can spontaneously cast it due to prefered spell. So prepare other spells for utility, and when they are not useful sack them for firebrand which is pretty much a combat ender. When you do cast firebrand it it is only a level 5 spell and you are high enough level you can cast level 7 spells. If you are a universalist you have 4 level 5 slots per day, 3 level 6 slots per day, and 2 level slots per day. Face enemies with fire resistance, will throw on a searing spell for a 6th level slot, or if the enemy has a high save dc throw on a persistent spell so he must save again.