Actually technically you can target the slann using war machines. If a person specifically sends a cannon ball, lobbed stone, bolt or some other template weapon. The slann will gain a 'look out sir!' roll and if you are unlucky to fail that one in six chance then at least you've got your ward save.
There are a couple of weapons like the empire hochland long rifle, and a few spells that can target a specific model in a unit. But for the most part, he is pretty safe and any cannons/templates that happen to cover him, he gets look out sir as mentioned above.
Bolt Throwers are a strange case, but nobody can ever claim a LoS roll from a bolt thrower shot. Instead, if the target unit has at least 5 rank and file troopers, then the bolt will automatically hit a regular trooper in the front rank (if the bolt comes from the front). However, this could create a problem for the Slann, since he may very well have a regular trooper in front of him. So stick the unit champion and maybe a scar-vet in front of your Slann and the bolt will have no chance to hit the Slann, unless it comes from the side or rear. T = Temple Guard C = Champion V = Scarvet S = Slann TTCVT TTSST TTSST TTTTT This first setup is safe from the front, since bolts will only hit regular troopers. It is still vulnerable from the side, the right flank in particular since the bolt would have fewer kills to hit something juicy. But if this were to happen.... TCTTT TTSST TTSST TTTTT Then I guess you'd randomize which of the regular troopers were hit, and half the time it would be a model in front of the Slann.
hochland long rifle still requires LoS since the slann is in the middle of the unit it will be out of LoS like a skaven character at the back of the unit.
::sigh:: I don't have my LM book with me but I'm almost 100% sure that a slann in a unit is considered a large target since he floats above the unit in order to cast spells. There by units which have the ability to pick characters out of a unit such as a HLR can, in fact, target 'That Toad' PS 100th post! Whoohoo!
In 6th and 5th edition yea he was a Large Taget, but in 7th they changed it so that if he was in a unit of Temple Guard he could Cast as if he was a large target, but is not a large target at any other time or when he is by himself. Wierd I know, they did some strange things in this last book.