First off, why 2,400? 2,500 is more the norm
Slan
Focus rumination
Beaming cogitation
cupped hands
Bsb
You're on the right track. 'Becalming Cogitation' really isn't that good, people will just stay out of range and hit you with longer-range spells anyway. I'd take 'Focus of Mystery' over 'Rumination', the free cast dice per spell is simply amazing. I'd recommend taking 'Unfathomable Presence' on him as well, it makes his Temple Guard highly resistant to magic. If you take a 'Plaque of Tepok' on him, you can get almost as many spells anyway. 'Cupped Hands' is a good choice, and I'd also make him the General.
Seeing as this is 2.5k, a bodyguard for your Slann isn't a bad idea. I like the following build;
Old-Blood w/Sword of the Hornet, Talisman of Preservation
Attach him to your Temple Guard, and he'll take care of most enemy champions pretty readily. He's also quite tough to kill, and due to the Slann's 'Unfathomable Presence', he's got a 2+ ward vs spells (great for blocking character sniping spells like 'Golden Hounds' or death magic).
Skink chief
Acient stegadon
Warspear
Just no. An Engine of the Gods gets you the same beast, but it's generating a shield, reducing spell cast costs, or generating a 360 degree laser beam into the enemy. A Skink Priest also lets you cram more anti-magic gear. Here is what I usually take;
Skink Priest, dispell scroll, diadem of power, EOTG
Might wanna downgrade them to 25, then use those points to contribute towards another block of Saurus. Two is enough to guard the flanks of the Temple Guard+Slann Deathstar, and they're pretty decent fighters in their own right, so investing in a sizeable number is worthwhile (especially when backed up with buffs or hexes from your Slann nearby).
10 skinks
10 skinks
10 skinks
10 skinks
All with blow pipes
I'd drop one unit, then use those points (as with before) to build the second Saurus unit. Otherwise, good choice, you avoided the Skink+Krox trap people fall into.
4 Terradon riders
4 terradon riders
I'd give these guys a miss to be honest. They're quite expensive but very easily taken out, and their 'Drop Rocks' ability turns them essentially into a bomb to drop on enemy warmachines. If you want anti-warmachine dudes, take a unit or two of Camo Skinks.
Drop the Champion (the Old Blood can take care of challenges much better), and take Banner of Eternal Flame on the Standard Bearer. Flaming attacks have a lot of applicability (cavalry don't wanna come near you, Undead and beasts also hate you), and it's extremely cheap. Your other alternative would be a Standard of Discipline, but Lizardmen don't usually have many problems with Leadership so I'd go Flame Banner.
Other than that, I'd drop one dude out to fit in the Old Blood, and take a Musician (so you can manoeuvre more quickly, and you can nullify enemy musicians).
Good choice, I'd maybe drop one of them to afford the above changes.
Overall, it's not a bad list, I'd probably drop the Terradons but the rest is just personal preference.