I see two potential big problems with this BBEG.
First his caster level is huge (30th, better than Demogorgon by the way). A group of 19th lvl, even 20th, has no chance to dispel his spell-like abilities, d20+20 with greater dispel magic against a DC 41 (and I think about his summon ability too...).
Also this makes his blasphemy spell-like ability mortal to the point of immediate TPK. I'm playing in 3.5, so baring spell resistance (32 with a spell resistance's spell at caster level 20, so 1 on 20 chance for him to not penetrate it, and automatic against 19th lvl characters for which the adventure is written for), 20th characters or less will be dead, no save in a 40-foot radius. 21-25th lvl characters will be "just" paralyzed for a few minutes, so certainly dead too.
Same problem with his word of chaos.
Also his implosive strike is huge. The DC is wrong. It should be, as a supernatural ability, 10+half 30 HD+Con mod (according to what is written in his stat block)=10+15+13=38 ! Which is almost an automatic kill, baring a really good roll on save.
I was thinking of using his strength for this ability (seems to make sense to me), which would reduce the DC to 33, which is still very high (and still higher than what is written). It looks like he will easily kill 3 characters with this. And since it's not a death effect, death ward is useless.
For those who have run this fight, how did it go ? Did you change anything ?
You could make it do the Pathfinder version of Implosion. It'd be 200 extra damage on a failed save, for Adimarchus CL 20. In practice, it's basically still going to be an instant kill, but using DMG numbers instead of death might offer some exceptional characters the ability to survive.
Alternately you mmight simply turn it into a Death effect, which can then be prepped against and stopped.
I don't know about you, but the first time I ran this the party of four characters which remained the same throughout the entire AP reached level 22 by the time they encountered Adimarchus. We agreed as a group to run things by the RAW and that's where we ended up. Epic level and everything, all the official WotC and Dragon Magazine source material for 3.5 was allowed which meant PCs had access to some pretty righteous abilities. It was an eternal fight which took an entire evening to be sure, but a total blast. When the chips fell the party had actually used up relatively few resources and from that perspective seemed rather anticlimactic. These were a bunch of min/maxers though. Things like extended greater spell immunity to blasphemy were SOP.