It's true that Teclis/Book of Hoeth tends to make frustrations run high among his enemies, especially given the big new boost to the power of magic in new rules. Thankfully, I spend a lot of my time fighting an opponant who uses Teclis, along with the Star Dragon lord and the 30 man unit of White Lions, and I don't lose all of the time.
It's fair to say that everyone hates him with good reason. My solution is to use units that have ASF. Teclis and his unit are likely to be strong, but if both sides ASF then Teclis, his assailant and his bodyguard will strike at the same time. Vampire assasins with the swift striking sword are good, actual Dark Elf assassins are even better. Teclis may have a lot of wounds, but unfortunately for him he just doesnt have enough to outlast most enemies for any length of time.
Alternatively, I crush his unit. This is easy for me to say, of course, given that Lizards have very strong core choices but Warriors of Chaos, Orcs, Beastmen and so on are all more than a match for spearmen. Things that do mass damage to the enemy like Curse of Years or Soul Stealer will also decimate elite units. If he's in a very strong elite unit like White Lions then... Well let's just say assassination is probably the best choice.
Also, I know that in theory I could take lore of life with my slaan and just throw 7 dice int Dwellers below and hopefully survive the miscast, but I don't like this solution. Firstly, I feel like it's just sinking to his level and relying on over-powered new magic rules. Secondly, he could just give Teclis lore of life and do the same thing to me. And finally, it's just not fluff worthy. Slaan are ancient and wise beings that have led their race for centuries, they don't act rashly and risk their own souls over some pesky warmblood.
The point is when I come across an enemy who uses white lion hordes, or irresistable miscasting Heralds of Tzeench or indeed over-strong characters like Teclis, I don't respond by using similarly short-cutting tactics. I want to be able to crush these tricks without having to plan my list out for them beforehand; I feel it's more classy that way.
Having said all of this, if you are a High Elf player and don't see anything wrong with it, that's fine too. I recommend, if you intend to field Teclis, you need to keep him off the front line at all costs. He should be in a small bunker unit behind that big phalanx of spearmen, maybe in a unit of Pheonix Guard. Also, Teclis is at his best when he is buffing his army or de-buffing the enemy. Consider giving him Lore of Shadow, for example. That way he can also escape if things are going very bad.
Finally, don't give in to the temptation to put him in an over-strong unit. Because your enemy will just devote their time to killing that unit. I once saw a match in which Teclis had a unit of 50 spearmen, Caradryel in the unit. He just hit the unit with -D3 Toughness and then attacked them with Salamanders and Chamo Skinks, and ended up killing every single one of them... And then Teclis too! Essentially, Teclis is a deadly and powerful character, but he's not going to win the game for you on his own! Even he's not that good.
So there is my advice both for killing and protecting Teclis.
"Don't call me small! I'll breakdown your feet and stick 'em on your head!!." Full metal alchemist