POWER WORD PAIN
Enchantment (Compulsion) [Mind-
Affecting]
Level: Sorcerer/wizard 1
Components: V
Casting Time: 1 standard action
Range: Close (25 ft. + 5 ft./2 levels)
Target: One living creature with 100 hp or less
Duration: See text
Saving Throw: None
Spell Resistance: Yes
A word you speak causes continuing pain to your target.
You utter a single word of power that instantly deals 1d6 points of damage to one creature of your choice, and
another 1d6 points in every round thereafter for as long as the spell lasts. The duration of the spell depends on
the target’s current hit point total, as shown below. Any creature that currently has 101 or more hit points is
unaffected by power word pain.
Hit Points | Duration
50 or less | 4d4 rounds
51–75 | 2d4 rounds
76–100 | 1d4 rounds[/sblock]
At level 4, you will face CR 4 monsters. Let's take a
Giant Crocodile, with 59 hp. You'll deal 1d6*7 damage, or 24.5 on average, over time with one casting. It will take 3 castings to finish it off, so the only second level spell slot you're left with is the one you get from high CHA.
You have to be with in 25 ft +5 ft/2 levels to cast it. That's 35 feet at level 4 if I'm not mistaken. You obviously can't run away after casting it since applying metamagic to a spell turns it into a full round action, so you are open to retaliation. Since a giant crocodile has a movement speed of 20 ft, it charge, double move, or run to cover distances of 40, 40, and 80 feet respectively. This make casting Expeditious Retreat and running away very difficult.
Best case scenario? You start off 35 feet away, cast Power Word: Pain, and run for it having cast Expeditious Retreat the previous round. You are now 80 feet away. Crocodile runs 80 feet and since it has a reach of 10 feet, it is in position to threaten you. This is a problem.
I'm not cherry picking monsters. Pretty much anything that is a CR appropriate melee creature is going to laugh at your attempts to annoy it to death and murder you.
And at higher levels you effectively reduce the amount of time in combat until really high levels
At CR 8, you already start seeing monsters with more than a hundred HP, such as the
Dire Tiger,
Elder Arrowhawk,
T-rexes,
Gargantuan Spiders...
Even at level 6, you'll be facing things like
7 headed Hydras (fast healing!),
Chain Devils (regeneration!),
Annis Hags (SR!),
Girallion (fast movers)...
This isn't even factoring in all the undead this game has which are flat out immune to Power Words.
In short, I do not think you have thought your cunning plan all the way through. A full plate, tower shield, silent Power Word: Pain build is not "OP" between the levels of 4, when it comes online, and 8, when it becomes obsolete. It does not solo enemies well, nor does it effectively contribute to a party as well as a sorcerer who is able to cast the highest level spells available to him and buff his friends, debuff his enemies, and have a lasting, immediate effect on the battlefield.