Well, let's start from the top and work down.
On the Slann, I'd say that Unfathomable Presence would be better overall then Becalming Cogitation. Nerfing one wizard isn't as good as granting MR 3 for the whole unit I think.
Core looks alright, along with your Specials. Your Rares are another issue I think.
As it stands, you've got a pure offensive unit and a very defensive unit. Ancient Stegadons are great for coming in and practically annihilating a unit on their own right. Just gotta remember to give it some kinda support. Razordons on the other hand, are amazing anchor units, holding down a flank, denying anything but heavy cavalry.
I think it'd be a better choice overall to drop one in favor of something else. Going more offensive, I'd say swap the Razordons out for Salamanders. Defensive, you could actually swap in 3 more Razordons for the Ancient Stegadon with points left over. Another Terradon, or some more Saurus warriors wouldn't be bad. Or even another Priest, just for the extra dispel and able to channel spells through him from the Slann.