Although Axolotl's suggestions on how to defend a lone slann is great, I would strongly disagree with his advice not to field a lone slann.
In my opinion, the more generations you add to a slann the more important it is to have him fielded as a lone slann. I would suggest this quite simply because it is my opinion that a slann is by no means safer in a saurus/temple guard unit then he is alone. Yes, you can create a 'rock solid' unit if you invest in various banners and upgrades, but at that point you are investing points on the assumption that your opponent will take the initiative in the battle. Ideally, your slann would never be in combat, but then you are wasting all those points. If you put those points to work, you are endangering the slann. No matter how great you make the unit, there are still the Chosen Knights and Blood Keep Knights out there, and the size and movement restrictions of the Saurus guard just decreases the chance that you can redirect the charge or manuver out of the charge arc. Being in the open also shows your slann to an enemies entire firing base. With only T5 he isnt going to last long unless you invest in a plaque of protection, which were points that could have been invested to make his magic phase more overwhelming (PoDom, Diadem, etc.).
If one places him out of LOS in a forest or other terrain, he can use the skink priests to get the LOS he needs for his targets. Theoretically, and generally speaking I have found to be the case, by placing him deep enough in the woods (2"), no opponent can charge him until they are within the terrain feature itself. This gives you one turn to deal with all the nastiness about, using the aforementioned tactics. If there is not a suitable piece of terrain around to camp inside of (or you are playing a WE player, etc.), keep in mind that the Slann is still quite manuverable, being able to move 8" after turning up to 360 degrees. Much like the king in chess, it is sometimes beneficial to move him around with a rolling screen if things get dicey. Keep in mind also, that with the 2nd generation slann, when an enemy unit gets into a charge arc, a focused turn of spell-casting should be enough to wipe out the unit.