If you just mindlessly stick the expensive TG/Slann out there and wait for every enemy unit to smash into it, of course it's going to die. If you base the rest of your army around the fact that you're running a Lifestar and properly support it, you will in many cases succeed. You have awesome augment spells, you have the Slann's abilities like Becalming to help shut down spellcasters, and you have every other model in your list waiting to help you.
Yes, they can hit it with stegadons/chariots/Bloodthirsters/ whatever else en mass, and depending on what buff spells are in effect, some, many, or most temple guard will die. (That's why I take 26 of them, minimum). But, in most cases, it won't be enough to destroy them all, and then the enemy is stuck in one of the single best tar pit units out there. And, presumably if you're running this unit in the first place, you now have saurus, skrox, or perhaps the odd stegadon of your own waiting to blitz them in the side.
A champion is a must have for this unit. He can deflect a lot of damage from the rest of the guard, he can have a neat item, and you get to resurrect him when you cast Rebirth on your unit- there's not a lot of downside. Again, Chakax is probably points better spent elsewhere, unless you're playing in an absolutely massive game, or you plan on facing WoC a lot.
The Lifestar does have weaknesses, as all gigantic battle stations do, and if someone specifically tunes their list to beat it you may have tough times. But, in a tournament situation, most folks are running blind 'all comers' lists, and your temple guard should do reasonably well.
It's not for everyone, but it's worked well above my expectations in the games I've played so far.