Rystil Arden said:
The Arcane Trickster and Marshal are lower-level cohorts (and the Paladin might as well be in a fight, although he's good at Diplomacy)
To do that 500 damage in your example, the entire party has to win initiative. Also, the fighter would likely not have hit both times without the +12 from the Warblade. Finally, expecting anything (particularly a CR 21 demon) to fail the save against a Disintegrate is a bit presumptuous (and don't forget SR 34).
As to the second encounter, they would have slaughtered it. Not enough AC or to-hit to matter (for comparison, Mammon had AC 41 and +42/+37/+32/+27,+35,+35) except for the Dragon, and DR the party can penetrate. If the marilith is out of reach but the hezrou are clumping up (I'm not sure how this is set up, exactly, but we'll assume that somehow they did), as many as he can reach will die when the Fighter gets his turn (which, based on the Hezrou's initiative bonus of +0, is guaranteed to be before all of the hezrou) thanks to that AC 23. As to the DC 24 Stench--Mammon had a DC 35 Poison, and that was closer to being right for the party. The only character with bad Fort is the Arcane Trickster, and she doesn't matter. Nobody else will fail against the Stench unless they roll very poorly, and the Ninja/Monk is even immune. The Hezrou can't hit anyone in the party with their Bite +14 melee. The Marilith has a palatable chance to hit, but she'd need to set up for a full attack to land attacks regularly (that is, more than once every four rounds or so), and if she takes a full attack from the Ninja/Monk and the Fighter, she's dead (the Ninja/Monk doesn't have as many attacks as the Marilith, but he's decently close, and they hit harder--plus they can all Power Attack full and still hit her AC). The dragon is the most useful of the bunch thanks to tolerable AC and HP, but he won't do so well alone.
Much to the designer's credit, the CR 21 Aspect of Mammon was actually an entertaining EL 21 challenge. And I do recognise that this is a marked change from most high-CR Outsiders, and I
completely agree with you that the MM high-CR Outsiders are worthless as lone opponents for the party.
Let's break this down by monster group.
HEZROU
Hezrou are Large, so the fighter presumably can reach two at most at any given time. Also, he can do 138 hp in a single attack (I assume you're talking about Cleave, which only grants one attack)?
Assuming the average party member can only fail the save against the Hezrou stench on a 1-2 - which makes them *extremely* save buffed for APL 17, I might add - then the chances of them making all their saves is only 43% (passing eight 90% chances). This requires them to have Con and magic item bonuses of +12. If they fail on a 4 or less - which is still above average, IMO, requiring Con and magic items to amount to a +10 bonus -, they have only a 17% (passing eight 80% chances) chance to make it through the encounter without failing once.

If they have what I would consider *average* fighter Fort saves (+18), they have only about a 2% chance of making all eight saves (passing eight 60% chances). I'd say an APL party even close to reasonable for their level will lose well over half their numbers to the hezrou stench. I suppose I can see the paladin of freedom making his saves (+10 base, +5 Con, +4 Cha, +4 magic items?) more often than not, and the monk is immune, but by rights the warblade and fighter should expect to drop just from the stench, and they appear to be the primary damage dealers.
The hezrou have a 10 ft. reach, so they never need to take a full attack from anyone smaller than them. You didn't mention the test characters' races, so I'm assuming they're human or at least Medium. The ninja should be largely hosed in this fight because they never need to take more than two attacks from him - one from his attack and one from his AoO.
Admittedly, barring dropping the whole party with Stench (ninja excepted) and slowly beating them to death on natural 20s (the whole party has AC 34 as well as Fort saves of +18?

), which is hardly out of the question, the hezrou probably can't kill any combat PCs.
MARILITH
On her own, she would maybe challenge one PC - which is proper, she's the same CR as them - but would probably be hosed against two or more. Fair 'nough.
However, she doesn't ever want full attacks. She wants to hit exactly once, and that with her tail. Her objective should be to grapple and constrict, because she should be able to reliably kill any one PC this way. Or do the PCs in question have Grapple bonuses of +29 in addition to being mass damagers, save tanks and AC tanks? (Admittedly, a Ring of Freedom of Movement would stop this).
The marilith would be the weak link here, provided she weren't riding the:
FIENDISH WHITE DRAGON
First of all, if the dragon gets a single action, no one need ever charge in this battle. Freezing fog acts as a solid fog (which would prevent it) *and* doubles as a grease spell over the entire area (which would also prevent it). I'm not clear if the grease effect would last after the fog disperses; if so, then that's practically an auto-win against a charge-focused group. Otherwise it just gives the fiends a limited tactical advantage and may not be worth using. I'm thinking it is because the hezrou will last longer, but their offense will be considerably reduced, too.
The dragon can deliver the marilith and himself into grappling range without ever provoking an attack of opportunity or being in reach of the PCs. This right here has to be dealt with (by a fly spell, presumably, which from the looks of it would be the domain of the arcane trickster; pity she 'doesn't matter' and is almost certainly downed if even two hezrou approach her). Otherwise, the range-lacking PCs pretty much have to flee or die - how else can they plink away hundreds of hp?