8th Ed. How offensively to use the slann?

My slann usually never dies unless I really goof up. 20 guards is IMO too little. I usually go with 30 or if I'm pressed for points 26. If you care for it give them the armour piercing banner so they can threaten armoured units.

Here's the thing though - don't just send in the unit if not necessary. This DOES NOT mean that you shouldn't do it at all, just don't rush in. Our guards are good, but not the best there is. You should also account for wounds before they strike so say you lose 4 models - that would leave you with 16 left. That unit won't last more than a single round in combat before it loses effeciency (attacks).

Here's what I do:

If points are there for it 30 guards with razor banner. This will scare off most since they'll stick around for long due to Ld9 stubborn + cold blooded. I'll put the unit where the leadership (slann) is needed.

I'll try to pit the unit against something reasonable - this means that if I see a witch elf horde I DON'T pit them against that unit.

I'll use 1-2 skink priest to have board control in terms of magic reach.

I'll have both the channel staff + channel discipline along with reroll failed dispel attempts and gaining a Power Dice from Dispel Dice. With 1-2 priests you can have a scroll and or the cube of darkness.
The goal is to reign supreme in your opponents magic phase so you can boost your own by converting the DD to PD. If you can do this you should on average come out with +2 PD, +1 DD and you'll generally be shutting down your opponents magic phase. Here's another personal note - with the ability to reroll a failed dispel you can allow yourself to be more risky with the dispel attempts (unless your opponent rolls ridiculously high). In return this'll make you dispel more even without the reroll meaning you'll be shutting down more magic than usually and thus gaining the upper hand. At least that's what I've experienced.

With Wandering Deliberation you've gained a wonderful tool box that can do some nasty things. For instance casting Iceshard blizzard and Mystifiying Miasma on an opponent will make sure that they won't be hitting much. Both a of low casting value.

Usually people will be scared of both spirit leech and searing doom, the other 2 damage spells? Not so much. So try and get those spirit leech and searing doom off against juicy targets and bait your opponent. Both of these spells (if they suitable targets are viable) will probably leave your opponent dry of dispel dice.
Now you're free to "nuke" crucial opposing units with the right combination of hexes and augments.

I have, for instance, held up a unit of grail knights with a single mounted scar vet because I reduced their armour save by 3 and hit them with iceshard blizzard. Hitting on 6's isn't all that fun and I managed to save the only 2 wounds that got through the wound phase. A charge from the britonnian elite knights should hurt. It didn't.

The exact same thing can be done with your templeguards. Hell if you see someone who's about to charge up your face and do nasty things to it even the odds so much that they don't dare or they'll lose out badly.

Both wyssans wildform (S6/T5 and amour piercing) or regeneration of 5+ will make sure these guys stay put. With both? A unit of White Lions will suddenly only do 5 wounds (assuming 6 wide and with champ) oppose to 8.

Here's the thing about it: In this specific scenario you can reduce the incoming wounds with various ways: wyssans + regen = 5. Wyssans + iceshard = 5. Iceshard + mystifying = 5 (against assuming white lions, 6 wide, 1 champ).

So the point is that you got plenty of ways to protect your outmost important unit. You don't necessarily need one specific combination if you want to protect your guards and with enough spells something WILL get off.

Just remember to know when to send in the guards with the premium frog. You don't want to bait something unreasonable that'll tear a big whole in the unit.

Hope this helps