With that criteria in mind... Ranger or Slayer can get you Shield Slam @ 2, and Shield Master @ 6 without prerequisites.
Outside of that... Combat Reflexes and Shield Focus/Shield Brace come to mind. If you continue, three levels of Phalanx Fighter might be worth it, depending on what you're going for.
Probably Vanilla Slayer, although Stygian isn't bad, and the archetype with shadow clones is neat... I just don't think either really help a shield and spear build.
I like the Freebooter archetype for Ranger... if you decide to take that route.
Kasatha seems like a good race for such a build, but it's not worth the disappointment of being told that your four-armed character is exactly the same as a two-armed character... but if your GM isn't lame, Kasatha can be fun.
1. Combat Reflexes
2. Shield Slam
3. Upsetting Shield Style
5.
6. Shield Master
Looks like you still have room for Power Attack or Iron Will or Improved Initiative or whatever other nonsense you may desire... Improved Shield Bash at level 5 is a smart choice, and sets you up for the next feat in the Style chain if that is something you want to pursue further.
My first thoughts are that the occultist doesn't need hands to cast, and that the "spear fighter" archetype is pretty good.
Some thoughts:
Occultist with Trappings of the Warrior panoply can have 6-level spellcasting and (effectively) full BAB.
As mentioned, ranger or slayer are good choices for the Two-Weapon Fighting or Weapon and Shield Combat Style feats.
The phalanx soldier fighter archetype is possibly worth looking at for at least a 3 level dip to pick up Phalanx Fighting ("At 3rd level, when a phalanx soldier wields a shield, he can use any polearm or spear of his size as a one-handed weapon. This ability replaces armor training 1.").
I guess the first thing you should nail down for your concept is "how much magic/spellcasting do I want?" The second thing you should ask yourself is "how much should I invest (feats, etc.) in improving my attacks with my shield?" The third question to answer is probably "what polearm should I use?" (the hooked lance is probably a good choice for damage [x4 critical multiplier] and is a reach, trip weapon; it's also a martial weapon, which saves using a feat to pick up an exotic weapon like the fauchard).
If you're a fighter you should just take the shield brace feat rather than phalanx soldier fighter. Doesn't screw with your weapon/armor training.
Does it have to be a spear, or could it be any polearm?
I would personally suggest a fauchard fighter (one case where exotic weapon proficiency isn't bad) with shield brace feat and power attack.
Now you have a two handed weapon with a good crit range and a shield. Invest in strength and power attack.
That's the basis of your build, and you can do all sort of things from that point.
Making a successful martial character in PF1 is basically as simple as high strength and power attack.
By level 6, you can afford the Ioune Stone that grants EWP, and be using a Fauchard... however, you don't want to spread yourself too thin.
Using a shield and spear requires its own set of feats. If you start adding on dumb $#!+ like crit-finishing or Trip (or literally any Combat Maneuver that isn't Dirty Tricks), you start running out of room for anything other than meathead combat feats. I are spear good, and my favorite food is boots, they taste like purple.
You can start with 3 levels of Phalanx Fighter, a level of Crusader Cleric for Improved Shield Bash and some Will save, then two levels of Ranger/Slayer for Shield Slam... and continuing towards 6 levels (character level 10) for Shield Master.
1. Combat Reflexes
1. Power Attack
2. Upsetting Shield Style
3. feat
4. Improved Shield Bash
5. feat
6. Shield Slam
BAB +5, +8/+4/+3 saves...
My first thoughts are that the occultist doesn't need hands to cast, and that the "spear fighter" archetype is pretty good.What Archetype is that?
If you want an effective spear+shield fighter using PoW - play a warder (no archetype). Get the pikeman's training feat, the piercing thunder style feat, and the maneuvers bronze lancet charge and iron lancer's edge. Charge using both those maneuvers, usually recover maneuvers with the warder's defensive focus the round after and deal out AoOs while doing the latter. Repeat. It'll work and be effective if boring.
I second (or third?) Trappings of the Warrior Occultist.
Feats: Combat Reflexes, Shield focus, Shield Brace. You'll be turning your +1 Spear into a +2 (enemy-Bane) Spear (BAB+STR+4 to hit, 1d8+2d6+4+(STR×1.5) damage) at the beginning of combat, so the damage bonus from Power Attack is actually likely to cost you damage since you'll hit less often. If you really want Power Attack (or some other feat) you can go Human.
Also the Haunt Collector archetype (with the Champion Spirit) gives you +2 damage and an occasional +2 to hit and damage for the round (in place of the usual resonant power of the Implement that's haunted).
Stats at level 1 are something like: S16, D14, C14, I16, W12, C7 pre-racial. You couod definitely optimize that better (but I left that fairly vague at L1 since I don't know what race you'll be using). Remember that your Transmutstion Implement should be acting as a +4 STR belt at level 6, so you should be great at hitting things.
I wonder if a 1-level dip to become a swashcultist would work? A spear can be a one-handed piercing weapon (see: doru) and even if you won't be getting panache from crits much, kills are still a possibility.