OK, let's start by assuming that the character is built at 20th level and therefore doesn't have to worry about what he does before he gets the skin of the Proteus. Fair is fair after all.
So: tallfellow halfling fighter 20
Str 7 (why bother, we've got the skin of the Proteus)+5 inherent, Dex 10 (again, why bother), Con 14 (no particular reason to bother), Int 14, Wis 14+5 for level increases, Cha 14
In Fire Giant form (we're allowing higher manifester level skins after all)
Str 42 (31 base+5 inherent+6 belt), Dex 15 (9 bast +6 gloves), Con 25 (19 base +6 bracers), Int 14, Wis 19, Cha 14
Feats: Power Attack, Cleave, Dodge, Mobility, Elusive Target, Improved Toughness, Improved Sunder, Combat Brute, Combat Reflexes, Stand Still, Combat Expertise, Improved Trip, Iron Will, WF, GWF, WS, GWS: Greatsword
Hit points: 274.5 (assuming that the form's con score is used for hit points as per the polymorph Rules of the Game article)
Skills: Autohypnosis: +18 (11rcc), Listen +18 (11 r cc), Ride +3 (1 r), Spot +18 (11r cc), Swim +5 (3 r) Tumble +2 (5r cc--can't be used in his armor but gives bonus to fighting defensively and full defense), Use Magic Device +10 (6r cc)
Saves: Fort: +29 (+12 base +7 con + 2 morale +1 luck, +1 insight, + 5 resistance, +1 halfling)
Ref: +18 (+6 base +2 dex +2 morale, +1 luck, +1 insight, +5 resistance, +1 halfling)
Will: +23 (+6 base +5 wisdom +2 morale, +1 luck, +1 insight, +5 resistance, +1 halfling +2 iron will)
Equipment:
Tome of strength +5 (137k)
+5 vest of resistance 25k
Luckstone 20k
+6 belt of strength 36k
+6 gloves of dex 36 k
+6 bracers of con 36k
+5 large holy cold iron greatsword (74k)
+5 adamantine fullplate of heavy fort, heartening (110k of so)
+1 parrying adamantine armor spikes (11k)
+5 animated heavy shield (64kgp)
Boots of Speed (12k)
Ring of Protection +5 50k
Amulet of Nat Arm +5 50k
Cape of the Montebank (10k or so)
Skin of Proteus (80 k or so)
Standard of Heroism (40k)
For a reduced cost, more min-maxed version, substitute 2 level 3 pearls of power (clvl 20) and a lesser metamagic rod of extend spell for the +5 enhancement bonus on his weapons and armor to free up about 80kgp in funds. More could be freed up by substituting pearl of power 2s (barkskin) and a rod of lesser extension for the amulet of natural armor.
Armor Class: 50 (10 base -1 size, +14 natural (+9 fire giant, +5 enhancement), +13 fullplate, +5 deflection, +1 insight, +1 dex, +7 shield), 51 with dodge, 52 with haste and dodge.
Grapple: +42 (+20 BAB, +4 size, +16 str, +2 morale)
Trip: +25 (+16 str, +4 size, +4 feat, +1 luck)
Attack: +44 (+20 BAB -1 size +16 strength, +5 weapon, +2 greater weapon focus+2 morale)
Full Attack: +44/+39/+34/+29 for 3d6+33+2d6 vs. evil
Haste full attack: +45/+45/+40/+35/+30
Offense Analysis:
Average damage per single/full attack (calculated by
http://www.distanceeducationconsultants.com/ddcalc.php)
Single attack (with power attack)/Full atk: Power Attack/Full Atk:No Power Attack
Against Generic monster (non-evil AC 35): 134.5/276/240
FRA with Combat Brute Momentum Swing and a six point (non-optimal power attack): 292
Against Balor (AC 35): 141.5/272.5/304
Against Pit Fiend (AC 40): 103.5/185.5/199.5 (Includes DR 15/silver and good)
Against Wyrm black dragon (AC 42): 109.5/237/237
This, of course, improves if we use the >50% chance=hit analysis (but I'm not sure we are). It also includes if we assume a round 1 charge and then add momentum swing damage from Combat Brute, the damage goes up noticably.
Among our test cases, the only one left standing at the end of round 2 is the wyrm black dragon--and having taken 346 points of damage or so, he's probably not too happy. The fire giant form fighter can afford to just stand there and trade full attacks with the dragon and he'll usually win.
Physical Defensive Analysis:
Vs. Generic +35/+30 for 3d6+20 damage: 8.25 damage/charge, 9.625/full attack
Vs. Pit Fiend: needs 20's to hit.
Vs. Balor: 20% chance to hit on charge attack; 10% chance to hit with longsword; needs 20's to hit at all when TWF
Vs. Dragon with multiattack: (+42 bite: 4d6+12, +40 2 claws 2d8+6, +40 2 wings 2d6+6, +40 tail slap 2d8+18) 51.65/full attack (Note that high AC and Elusive Target are likely to help here since the dragon really can't get anything out of power attacking).
Vs. Grapple: Can easily pin anything short of the wyrm dragon or the tarrasque. Is usually too large to be snatched by a black wyrm and can be too large to be snatched even by a collossal dragon if he wants to be. (And really, the snatch feat is the only kind of grappling most dragons do willingly).
Vs. Multiple opponents, he has AC, cleave (which will increase his take from 1 Cornugon or Marilith per round to two), Elusive Target (which would be helpful when facing multiple CR 20 dragons or demons), Combat Reflexes, and Stand Still which are quite helpful for battlefield control.
Flexibility of attack and defense:
For added humiliation to the balor, he can easily sunder both its whip and its longsword without reducing his damage/round in the slightest.
Trip, Combat Reflexes, and Stand Still give him a lot of options against opponents with less reach than him. These could be exaggerated by using a reach weapon (switching to a glaive would reduce his average damage per hit by 2.5 and switching to a guisarme would reduce it by 4.5--neither of which is particularly significant). Switching to a spiked chain would reduce his hit points by 20 (swap Imp Toughness for Exotic Weapon Proficiency) and his damage by 4.5/hit but the increase in reach and flexibility would probably be worth it. (If he used any reach weapon, the dragon could no longer execute fly-by attacks and bite him without provoking AoOs and large foes like the balor and pit fiend would generally provoke AoOs if they wanted to close with him and attack in the same round (otherwise, they'd just teleport)). Unfortunately, none of these kind of options--which IMO are what separate fighter from barbarians, etc--really show up in this kind of a head to head contest, so I chose to limit them and use a greatsword to demonstrate sheer damage-dealing potential (well, actually a falchion would be even more damaging but it's crit dependent and the greatsword is more reliable).
Skin of the Proteus enables him to take the form of an annis hag (reduce strength by six) in order to gain improved grab (very helpful considering that, even with the reduced strength, he'll still be likely to pin a pit fiend with a full attack grapple)
He is vulnerable to cold in fire giant form but can take Frost Giant form for immunity to that at slightly reduced strength.
He can take the form of a stone giant or Gray Glutton for superior defense and if he feels like using real cheese, could take the form of an ettin and wield a wounding weapon in each hand to get nine attacks per round for one point of con each). A lot of DMs wouldn't allow it but the rules call the ettin's superior TWF an extraordinary special attack. He has a variety of forms (Remorhaz, giant eagle, dragon, etc that he can take for mobility). An Athach would be a very good form for multiweapon fighting and could theoretically get in 12 attacks with wounding weapons (or whatever) if he could come up with Improved Multiweapon fighting and greater multiweapon fighting feats somewhere.
Other than flexibility, the Skin of the Proteus gives the fighter about +6 strength vs. what would otherwise be the maximum possible strength for the fighter, more con than he would otherwise be able to afford on point buy, more armor class than would ordinarily be possible (natural armor--though my first candidate could match it using Alter Self), and reach. The skin is probably more abusive on a Monk, PsyWar, cleric, Eldritch Knight, or multiclassed fighter than on a single-classed fighter for the simple reason that single-classed fighters don't get a whole lot out of their mental stats and therefore don't benefit as much from treating strength, dexterity, etc. as dump stats.
There's also Aasmar/Tiefling fighter 19
He uses the skin of the Proteus to assume the form of a planetar. The results will be similar but planetar is a more defensive form and can fly. Unfortunately, mariliths have 16 HD or they would be the ultimate multiwounding weapon cheese form.