I agree; the lack of hit points is a real issue, and a strange design decision when you consider that she has six weapon-wielding arms.
Try giving her the battle touch feat from BoEMIII which lets her hold a touch spell for a full round... which means that her inflict serious wounds spell-like ability, combined with project image, may have become very, very ugly indeed.
Yep, that would be six melee TOUCH attacks....
Oh, and here's a marilith with six Ftr levels and a scissors sword from Savage Species:
MARARTHANE, HANDMAIDEN OF SSETH
Female variant marilith Ftr6: Large Outsider (Chaotic, Evil); CR 23; HD 9d8+54 (outsider) plus 6d10+36 (Ftr); hp 164; Init +8 (+4 Dex, +4 Improved Initiative); Spd 40 ft; AC 31 [35 from unholy aura] (-1 size, +4 Dex, +18 natural [+4 deflection bonus from unholy aura]); BAB: +15/+10/+5; Atk: (OPTION 1) +27/+22/+17 melee (2d8+33 [+28 Str, +2 Weapon specialisation, +3 enhancement]/17-20/x2 plus Grapple, +3 keen scissors sword) and +21 melee (4d6+4 plus Improved Grab/20/x2, tail slam) or (OPTION 2) +21/+17/+12 melee (2d6+14 plus 2d6/19-20/x2, +2 unruly greatsword) and +26 melee (1d8+8/19-20/x2, +2 everbright jumping longsword) and +26 melee (1d8+8/17-20/x2, +2 keen longsword) and +26 melee (1d8+8/19-20/x2, +2 longsword) and +25 melee (1d8+7/19-20/x2, +1 longsword) and +21 melee (4d6+4 plus Improved Grab/20/x2, tail slam); Face/Reach: 5 ft x 5 ft/10 ft; SA: Spell-Like Abilities, improved grab, constrict 4d6+7, Summon Fiend; SQ: acid immunity (variant ability), cold resistance 20, damage reduction 20/+2, electricity resistance 20, fire resistance 20, poison immunity, telepathy; SR 25; AL CE; SV: Fort +18, Ref +11, Will +13; Str 26, Dex 19, Con 23, Int 18, Wis 20, Cha 14.
Skills: Bluff +14, Concentration +15, Hide +10, Listen +24, Move Silently +12, Scry +14, Search +14, Sense Motive +16, Spellcraft +14, Spot +25. Racial Bonuses: +8 to Listen and Spot checks.
Feats: Battle Touch, Cleave, Improved Initiative, Multiattack, Power Attack, Quick Draw, Weapon Focus; longsword, Weapon Focus: scissors sword, Weapon Specialisation: longsword, Weapon Specialisation: scissors sword. Bonus Feats: Multidexterity, Multiweapon Fighting.
Constrict (Ex): Mararthane deals 4d6+12 points of damage with a successful grapple check against Medium-size or smaller creatures. The constricted creature must succeed at a Fortitude save (DC 24) or lose consciousness for at long as it remains in the coils and for 2d4 rounds thereafter.
Improved Grab (Ex): To use this ability, the marilith must hit a Medium-size or smaller opponent with its tail slam attack. If it succeeds, it can constrict.
Spell-Like Abilities: At will- animate dead, bestow curse, chaos hammer, cloudkill, comprehend languages, darkness, desecrate, detect good, detect law, detect magic, inflict serious wounds, magic circle against good (self only), magic weapon, project image, polymorph self, pyrotechnics, see invisibility, shatter, telekinesis, teleport without error (self plus 50 pounds of objects only), unholy aura, and unholy blight. These abilities are as the spells cast by a 13th-level sorcerer (save DC 12 + spell level).
Summon Tanar’ri (Sp): Once per day a marilith can attempt to summon 4d10 dretches, 1d4 hezrou, or one nalfeshnee with a 50% chance of success, or one glabrezu or another marilith with a 20% chance of success.
Possessions: +3 keen scissors sword, +2 everbright jumping longsword (the weapon is always bright and shiny and completely immune to corrosive and rust attacks; twice per day flare with blinding light within 20 feet burst with all but the wilder needing to make a Fortitude save [DC 20] or be blinded for 1d4 rounds; featherfall while grasped; every other round, make a Jump check with a +30 bonus), +2 keen longsword, +2 longsword, +1 longsword, +2 unruly greatsword (marked with the symbol of Shaundakul and engraved with chaotic winds; once per day surround the wielder with a wind wall as if cast at 5th level)
Tactics:
Mararthane always has magic circle against good (+2 deflection bonus to AC, +2 resistance bonus to saves against good opponents only; duration: 12 minutes/120 rounds) and see invisibility (duration: 120 minutes/1,200 rounds) in effect. Mararthane particularly enjoys combat and is completely unafraid of entering melee, trusting her phenomenal armour class and enormous damage potential to see her safely through. Her tactics are thus generally simple unless she is aware that the enemy is particularly potent. In that case, she is likely to simply teleport away and await the enemy in a more advantageous location allowing her to use the Ranged Attack Tactics mentioned below.
Melee Attack Tactics:
1. Casts unholy aura on self (+4 deflection bonus to AC, +4 resistance bonus to saves, 1d6 points of Str damage to successful melee attackers [Fort save against DC 20 negates]) This lasts 12 rounds and does not stack with magic circle against good but the latter lasts longer.
2. Mararthane then simply enters into combat, using the scissors sword against a single opponent or a pair or trio, and all six longswords against a larger band.
3. Telekinesis may also be used to move a single opponent weighing up to 300 pounds into a chasm or pit (Will save against DC 17 negates).
Ranged Attack Strategy:
1. Casts unholy aura on self (+4 deflection bonus to AC, +4 resistance bonus to saves, 1d6 points of Str damage to successful melee attackers [Fort save against DC 20 negates]) This lasts 12 rounds and does not stack with magic circle against good but the latter lasts longer.
2. Uses project image to close with the enemy.
3. From the projected image, Mararthane combines the Battle Touch feat and the spell-like ability, inflict serious wounds, allowing her to make eight melee touch attacks (+23/+18/+13/+23/+23/+23/+23/+23) each inflicting 3d8+12 points of damage.
4. Uses to teleport to close with the enemy lashing out either with her scissors sword or longswords depending on the numbers.