I'll throw together stats for that psychic warrior build for comparison's sake, since again, I just feel like it.
However, a 5th-level psychic warrior cannot augment Expansion to 7 power points for the 2-degree size increase; they can only increase their size by 1 degree (Overchannel would only allow 6 power points to be spent at the time).
Dustwalker, half-giant psychic warrior 5, ECL 6/CR 6
Str 20, Dex 12, Con 16, Int 8, Wis 15, Cha 8
HP 35 (supposing half the maximum from each hit die)
AC 19 (touch 11, flat-footed 18), Initiative +1, BAB +3, speed 20 feet
Fortitude +7, Reflex +2, Will +3
Feats: Power Attack, Psionic Weapon, Weapon Focus (Greatsword), Reckless Offense, Improved Sunder
Skill: Concentration +11
Racial Traits: Giant, low-light vision, fire acclimated (+2 on saves against fire spells and effects), powerful build, naturally psionic (2 power points, already factored in below), psi-like ability (Stomp, 1/day, 2nd-level manifester, Reflex DC 10)
Class Features: Psionics (14 power points, manifester level 5th, save DC of 12 + power level, Expansion, Force Screen, Vigor, Animal Affinity, Concealing Amorpha)
I leave out Dissolving Weapon (in place of Concealing Amorpha) cuz it would be too cheesy and might very well insta-kill a PC, when combined with Psionic Weapon and a Huge greatsword and everything else.
Possessions: Large Greatsword +1, Masterwork Full Plate, Potion of Haste (8th-level caster), Potion of Shield of Faith (1st-level caster), Traveler's Outfit, 2 Belt Pouches, Flint and Steel, 44 gold pieces, 20 silver pieces
Attacks: Large greatsword +10 melee for 3d6+8 damage, PA(full) large greatsword +7 melee for 3d6+14 damage
Attack Notes: Add +4 on attack rolls against items on other creatures, does not provoke an attack of opportunity when attacking an opponent's weapon, can take a -4 AC penalty for the round with an attack or full-attack action in melee to gain +2 on melee attack rolls, can expend psionic focus to add +2d6 damage with one melee weapon attack.
Would start out hiding while buffing up. 1st round manifest Force Screen at 1pp for +4 AC. 2nd round manifest Animal Affinity at 3pp for +4 Strength. 3rd round manifest Concealing Amorpha at 3pp for 20% concealment. 4th round manifest Vigor, augmented to 5pp for 25 temporary hit points (putting total hp at 60). 5th round drink Potion of Shield of Faith. 6th round drink Potion of Haste. 7th round manifest Expansion at 1pp for Large size (powerful build making it effectively similar to Huge). At this point, only 1 power point remains in reserve, for use of psionic focus/psionic weapon.
8th round move out from hiding to go confront the party, heading in to attack. Might reach the nearest PC during this round, with Haste's speed increase. 9th round begin the series of full-attack actions with Reckless Offense and full Power Attack, using Psionic Weapon on the first attack. Probably target the party's warrior's weapon first, trying to destroy it with Improved Sunder, Psionic Weapon, Reckless Offense, and full Power Attack. After that weapon is destroyed, pummel the warrior into unconsciousness then move in against each of the next few targets, charging each one in turn with full-attack actions until they're unconscious.
Results:
1st attacking round: initiative is rolled at +0 after the Dex penalty from Expansion, total AC is 20, total HP is 60, total saving throws are unchanged, has 20% concealment, size is Large, attacks at +12 for 4d6+2d6+18 damage (probably breaking the opponent's weapon, if targeting it) and then a second time at +12 for 4d6+18 damage (probably directed at the opponent themselves, unless their weapon is still intact). Attacks are at +16 instead if striking the opponent's weapon.
2nd attacking round: attacks twice at +12 (or +16 versus opponents' weapons), dealing 4d6+18 damage each time.
3rd attacking round: repeats the 2nd attacking round.
4th attacking round: repeats the 2nd attacking round.
5th attacking round: 5-foot-step away and manifest Expansion again at 1pp, as it will have run out at this point (possibly sooner).
6th attacking round: get closer again if needed, and resume sequence from the 2nd round. Soon the Shield of Faith and the Haste will run out of duration, then the second Expansion will too. After that point, AC goes down to 19, size becomes Medium as normal, attacks are made at +11 instead of +12, and damage will go down to 3d6+17. Won't get 2 attacks per round anymore once Haste ends. The other self-buffs will last a few minutes longer, not that it's likely to matter.
Grag's HP are 1 higher when Raging, but drop to being 11 lower instead, afterward, since the extras aren't temporary HP like Dustwalker's HP from manifesting Vigor. Grag's AC will be several points lower much of the time, but he will have Diehard for a while. Grag needs no preparation time, unlike Dustwalker, who is vastly inferior if he doesn't get his half-dozen-odd rounds of self-buffing before the fight (though he can make do with just a few rounds of buffing and be almost as strong as normal, but it's a bad idea for him to waste more than 1 or 2 rounds buffing up if the battle starts before he gets any prep time). However, Grag would probably chug a Potion of Haste before the battle too, to make quicker mincemeat of his foes.
Grag, at full steam, would be attacking three times per round, at +13, +13, and +8, dealing 2d6+27 damage with each hit. Averaging 99 damage per round (66 if the third attack misses), compared to Dustwalker's average of 60 per round (66 in the first round thanks to Psionic Weapon). If Grag finds his third attack missing, he may lower the Power Attack value from full-6 to half-3, striking at +16, +16, and +11 each round, most likely hitting then, putting his average damage at 81 per round, still superior to Dustwalker's.
I think Grag wins.
If the two fought each other, Grag's Diehard from Bear Totem with Rage would allow him to survive the first round of attacks by Dustwalker, then he'd most likely turn Dustwalker into paste with his own full-attack. Though I'm not sure if Grag could chug one of his Cure Light Wounds potions while Raging, before he dies of losing the extra HP from Rage.
