The Wailing Banner is a bit of a waste on a Slann imo. Terror only causes a panic test if you charge an enemy (not if you get charged). You're not going to be charging into Close combat with your Slann (I really hope!), so a Terror-causing Slann has absolutely no benefit.
There's the Plaque of Dominion, which forces all enemy wizards that can draw LOS to it to make a stupidity test each turn. I've had some success with that before. If you only manage to shut a wizard (and his unit) down for 1 phase it's been worth 50pts, tho with the new BSB rules allowing re-rolls for stupidity tests it may not be quite as effective in 8th.
For solo Slanns the Sun Standard is very useful. Ranged attacks suffer -1 to hit (-2 if within 12"). Depending on the army you're facing; it's obviously a lot more benefit against a sneaky elf army than Beastmen or WoC, who have practically no ranged unit - Obvioulsy if you take Higher State of Conciousness 99% of ranged attacks won't be a threat anyway.
Flavour of the month seems to be the Standard of Discipline, which gives the Slann +1 Ld, giving all units within 12" a Ld of 10.
My fave combo for a solo slann is;
Flying Slann of Stupidness
Slann Mage-Priest - BSB
Arabian Carpet (flying)
Cupped Hands
Standard of Discipline
Becalming Cogitation, Focus of Mystery, Rumination, Higher State of Conciousness
Immune to non-magical attacks, can fly to avoid most combat and position within 24" of enemy wizard to make use of Becalming and Cupped Hands (hate when you miscast and there's no wizard in range)
