Sir ThornCrest said:
marilith have dr20/+2
I was just quoting what I remember he has as far as feats.
we havent encountered a +3 weapon yet so we havent had that come up I was just using that as a "I cant believe the damage this guy does statement"
but it is nice to know that you nead a = of magic to sunder I was still playing it just gave the weapons a bonus to hardness and hitpionts.
I gave the merilith 2 +2 b-swords of wounding, a repeating cross bow of speed w/6 dc17 piosonous bolts -1d6str, a +2 spiked buckler, and 2 +1 long swords and 1 flaming spear +1
so she was tough but not enough.
Thorncrest
As Ankh-Morhpork Guard (did I spell that right?) pointed out, you are using a 3.0 Marilith here. 3.5 damage reduction is significantly different, and Mariliths have DR 10/Good and Cold Iron. That's 10 less damage per swing. In addition, Mariliths have base AC of 29, should always have up Unholy Aura (AC 33), and have Combat Expertise with BAB to spare to increase that AC. You're Stone Child should have been hitting less than 50% on the first swing. Oh yeah, a 3.5 Marilith has over 200 HP and can easily do > 100 HP of damage in a full attack, plus has Improved Grab and a grapple bonus of +29.
Anyway, aside from the various rules discrepancies that others have pointed out, your problem seems to be "How do I handle a fighter who deals out tons of damage and can take a lot of punishment?" I think that I can talk about this with a little bit of experience, as I have a 1/2-demon fighter4 /barbarian6 in my campaign specialized in the fullblade, and he wields a +2 keen fullblade. His strength is 30 when raging, his AC is 25, he has boots of speed so is always hasted, and he is seriously a combat monster. Despite all this, he rarely outshines the rest of the group. For reference, I have another 14th level divine warrior (homebrew PrC) in the group who has 170 HP (!). Then there's the Bard1/Paladin12 riding a griffon who goes around Spirited Charging and Smiting with Divine Might. He's dealt over 90 HP of damage on a single charge in consecutive rounds. These warriors are the core of the muscle in the group, and each has strengths and weaknesses, and from one battle to the next a different one will have his day in the sun. But that's true of the other characters, too. In one of your posts, you mention a battle with 10 frenzied berserkers where a Force Wall saved the party. Without the wizard, your Stone Child would have just been so much meat, so maybe you need to make sure to point out the contributions the other characters make.
I had similar concerns to yours when the half-fiend joined my group, but I had to put it in perspective. The role of the fighters is to dish out damage. The role of the fighters is to kill things. Other characters help make the fighters' job easier, and they can kill things too, but it's not their primary role. For the party to succeed, the fighters have to be good at their job. At the same time, the other characters have to be good at their jobs. Let me give you an example from my most recent session:
The party was in a large ice cave, confronted with an invisible sorceress (lvl 12) flinging Freezing Spheres, about 5 Frost Giants with various fiendish grafts, a couple of Frost Giant Zombies, 6 Winter Wolves, and a maximally advanced Winter Wolf (the Den Mother). The paladin had been unable to bring his griffin into the cavern, so he was relegated to meat shield duty. The divine warrior was out of commision as he had been barred from entry by some unholy force. The half-fiend dished out some damage, but Frost Giants have HP to spare, and they can dish it out in return, so he was quickly on the defensive. If the Bard hadn't been able to confuse the Winter Wolf Den Mother, and the sorcerer and druid hadn't lobbed repeated fireballs and flame strikes, the three combat brutes in my party would have been creamed. The battle was EL 16 and I have 7 PC's of level 13-14 plus NPC support, so they should be fighting battles of EL 15 or so on average, so this encounter wasn't designed to be particularly overpowering. But it was challenging, because there were multiple foes, the foes' tactics were varied, and they could match each of the party's strengths with strength of their own.
I guess what I'm trying to say is that you shouldn't be too quick to overreact. There's a good chance that the character is just effective in his role.
Not that anyone cares, but next time we play I get to pit my group against a Gelugon, a Hamatula, two Hellish Rippers (from Hellstone Deep), and two Corrupted Frost Giant Skeletons with Fiendish grafts.

Now
that should be a challenging battle.