Paizo BLASTER CASTER build:
Goal: Pile on the hurty-hurt with direct
damage spells. You don't need battlefield control if the enemy is
dead.
Level 1: Take Sorceror: Crossblooded Orc/Draconic, Human, take
Varisian Tattoo, take Trait: magical Lineage (pick spell), Reactionary
(+2 Init)
Then take Wizard/Evoker -Admixture Specialist for your remaining
levels. Why Admixture? Because you can change the element of any of
your blasting spells on the fly to get around elemental
resistances/immunities.
If you want to superspecialize outside of Evoker, take Sin Magic, lose
two schools (Conjuration/Abjuration), gain yet another spell slot per
level of raw power.
Key Feats: Intensify Spell, Empower Spell, Quicken Spell, Spell
Perfection, Spell Penetration, Spell Specialization, Greater Spell
Specialization.
End result: Crossblooded sorc orc/dragon bloodline 1, Wiz/19,
Admixture Evocation specialist. Note: Can use Sin Magic for more
slots. Sorcerer level allows use of spell devices from denied schools
(Conj/Abjuration). Magical Lineage Trait allows Intensify for free on
chosen spell. Spell Perfection allows free Quicken at higher levels.
Play Hints:
Take Burning Hands or Magic Missile as a Specialized Spell early.
Burning Hands will deal more damage, Magic Missile has better
long-term utility and keeps you out of danger. Every other level, you
can change your specialized spell.
Add Greater Spell Specialization at level 7 or 9. Why? You can then
memorize utility spells, and trash them for your blaster spell.
Change your specialized spell up to Scorching Ray or Fireball when you
can, depending on campaign, typically at 5 or 7.
At level 12 or higher, change it to Fire Snake. Use Admixture
specializing to change the element on the fly.
Use Fire Snake until higher levels. Why? High damage base and level 5
spell still leaves room for metamagic, esp Quicken.
Mechanics behind Choices: Orc blooded, Draconic: +1 to all damage
spells, +1 to element of choice, retasked by Admixture = +2 dmg/die on
blasting spells.
Feats
Intensify Spell: Increases caster level damage cap +5 to apply
to a specific spell. Burning hands goes up to 10d4+20. Magic Missile
goes to 7d4+14. Fireball to 15d6+30, Fire Snake to 20d6+40.
Empower Spell: Increase dmg by 50%.
Quicken Spell: Hit enemy with two spells/rd.
Spell Specialization: +2 to caster level with a specific spell. Helps
bring the damage earlier and faster.
Varisian Tattoo: +1 to Caster level with a specific school (Evo). This
buys off your sorc level.
Greater Spell Specialization: Sacrifice spells to power your chosen
blaster spell. Means you can memorize utility spells freely.
Spell Perfection: Doubles fixed feat bonuses, apply one metamagic for
free. An Empowered/Intensified spell with Magical Lineage is still its
original spell slot. SPell Penetration doubles to +4. Varisian Tattoo
to +2. Spell Specialization to +4. Effectively, you've got +10 on
Spell Resistance rolls, and are casting 'Your spell' at 5 levels
higher then your own.
Top End Damage: 30d6 +60 from Fire Snake, empowered, Intensified,
average 165 dmg, save 1/2, level 5 slot. Quicken for another hit, 5th
level slot, 20d6 + 40dmg, avg 165.
= 330 blasting dmg in one round, save for 1/2. If you've a Rod of Maximize, you can lift this to 215 base damage.
Base level 5 spell slot is 20d6+40 dmg, 165 dmg.
By Level
At level 1, your Burning Hands should be 2d4+4 (avg 9)
At level 2, its unchanged.
At level 3, Spell Specialization kicks in. 5d4+10 (22.5). This tops it
unless you Intensify it.
At 4th, 6d4+12 (27), Intensified BH.
At 5th, Intensified BH, 7d4+14 (31.5).
At 6th, you can shift Spec to Scorching Ray. 2x 4d6+8 (44). Your
Fireball is 5d6+10 as well, or 7d6+14 if specced.
At 7th, 6d6+12 fireball, or 8d6+18 if spec. An Empowered, Intensified
Burning Hands, if still the spec spell, is 13.5d4+27 (about 60).
8th - Empowered Scorching Ray, 2 x 6d6+12. E/I BH is 15d6+30 (74, max)
9th - Intensified, Specialized Fireball is 11d6+22 (51).
10th - Firesnake. E/Spec Fireball is 15d6+30 (74). Emp Scorching Rays
are 3 x 6d6+12, or 18d6+36 (99 dmg). You can now Quicken a Burning
Hands or Magic Missile as kicker damage in a round, although you've
few slots.
12th - E/I/Spec Fireball is 21d6+42 (115) damage. You can now Quicken
a 12d6+24 Scorching Ray.
14th level - A Specialized Firesnake now exceeds/equals an intensified
Fireball. Fireball caps at 22.5d6+ 45 (123~) damage. An Empowered
Firesnake is 24d6+48 (132) damage. You can now Quicken a 10d6+20
Fireball. Intensified, Empowered Scorching Ray tops out at 24d6+48
(132 dmg)
15th level - Spell Perfection. You can now add a Meta for Free. This
will be Quicken or Empower. Intensified might be free if Magical
Lineage applies to it. Caster level buffs for spec spell exceed +5, so
top out at 20d6 dmg at 15th level. You will miss Spell Resistance
rolls against CR appropriate enemies on a 1. Assuming Firesnake, you
can now cast a Quickened Intensified Firesnake for 20d6+40 (110)
damage out of a 5th level slot, and an Empowered Firesnake out of the
same slot for 30d6+60 (165) damage. Using a 7th level slot, you can
Empower both.
16th+ - Damage remains the same, higher level spell slots are open for
use of other Metas or control spells.
Conceivably you could use Disintegrate to get a higher damage total,
but the delay isn't worth it, and you'd lose the Varisian tattoo
bonus.
If your DM allows you Twin Spell from 3.5, you can very, very easily
clock in at 495 raw dmg/round. IF he allows Arcane Thesis, god help
your enemies.
Best Answer
Caveat the First: I Am Making Broad Generalizations
There are cases where you might want to break these.
Caveat the Second: I Am Not Overly Familiar With Pathfinder
I have read the Core rule book, but none of the supplemental material, including the archetypes (alternate class features in 3.5 lingo), and Paizo did make it a goal to counter, at the very least, my point #2. Within the Core material that I have read, I would argue that they largely failed. I have heard that they have done somewhat better with the supplemental material, but I would not know. Mostly, the answer is really only being given for 3.5, with the understanding that Pathfinder Core changed very little in any significant way.
Caveat the Third: I View Classes as Purely Metagame Concepts
I have little to no problem with multiclassing, even a great deal of multiclassing, because in my games a character does not think of himself as a Paladin 6 or a Barbarian 2/Rogue 1/Cleric 1/Swordsage 2 – he thinks of himself as a knight, or a mercenary, or a mystic, or whatever.
Or, say, a Monk 2/Paladin 6 thinks of herself as a Samurai.
Each class is, as far as I am concerned, a bundle of mechanical features with some suggested ideas for what sort of person would have them. Players at my table know, for instance, that they don’t even have to ask if they want to treat, for instance, Rage as instead “Zen Focus,” so long as the mechanics do not change. Most abilities are reasonably generic and can be understood as a lot of possible things in character.
Multiclass characters can represent a change of heart – someone who used to train one way abandoning it for another – but particularly when you start at levels above 1 and you start with levels in a few classes, it often makes more sense to think of a multiclassed character as someone who pursued one path their whole life – that path is just mechanically represented by different classes.
1. Spellcasters should not multiclass
Spellcasters, as well as manifesters, will always benefit most from getting the highest-level spells (powers) available at a given level. Their spellcasting (manifesting) progressions benefit only from more (effective) levels in their class.
Note that technically in 3.5 jargon, prestige classes do not count as multiclassing, but a separate thing. Prestige classes can be very good for spellcasters, since they frequently have few or no class features aside from their spellcasting, and many prestige classes advance spellcasting on a 1:1 basis. In some cases, this can literally be “something for nothing,” as the prestige class provides class features while not costing a spellcaster anything from their base class.
2. Mundanes should multiclass
Mundane classes (and the weaker half-casters, usually the ones who stop at 4th-level spells) do not have a solid progression like spellcasters do. Furthermore, most of them are extremely front-loaded. For examples:
And so on. So a Barbarian 2/Fighter 2/Monk 2/Paladin 2 has way more class features than a Barbarian 8, Ranger 8, Fighter 8, or Paladin 8.
Note that Cleric 1 is quite probably the best single-level dip in the game, despite also being a fullcaster that you can focus all 20 levels on. In this sense, Clerics are both martial and magical. Even characters without enough Wisdom to cast Cleric spells can make good use of a Cleric dip.
Psychic Warriors are another exception: if you aren’t absolutely requiring full BAB, Psychic Warrior 2 can get you the same feats that Fighter 2 can, plus a few Powers which can be very useful to an otherwise-mundane warrior.
3. Those in between can go either way or, sometimes, halfway between
You can dip Bard for Bardic Knowledge, Inspire Courage, and fascinate, or you could focus on Bard to get its quality spellcasting. Binders (Tome of Magic) and meldshapers (Magic of Incarnum) can be dipped for a select Vestige or Chakra bind, or focused on to maximize those features.
Factota (Dungeonscape) go even further and are solid at 1 (all skills in class), 3 (Brains over Brawn), 8 (Cunning Surge), or 20 levels. Binders and meldshapers are also reasonably good at being worth however many levels of them you want to take.
4. Tome of Battle is exceptional and unique
Tome of Battle classes multiclass better than any other classes in the system, because they add half their level in other classes to their Initiator Level, and can select higher-level maneuvers based on this improved Initiator Level. That means unlike a Fighter 8/Wizard 1, who gets 1st-level spells, a Fighter 8/Warblade 1 has Initiator Level 5 and gets 3rd-level maneuvers.
Ardents (Complete Psionic) have a somewhat-similar mechanic for determining what level of power they can use, but no built-in bonuses to their Manifester Level from other classes. The Practiced Manifester feat (also Complete Psionic), however, means that an Ardent can take up to 4 levels in other classes while maintaining full Manifester Level and therefore the highest-level powers.