First off, I fully agree with the previous views that the chosen-star (CS) is best left alone or delayed by feeding it cheap units to redirect it. With so many points in the unit (well over half the army with the wizards), this approach will allow you to go after the rest of the army while only sacrificing a few skinks.
However, there is a chance of dealing with the CS in combat if you can weaken it prior to it gaining the 3+ ward save and stubborn (i.e. within the first two rounds usually). Even with the save, it can be tackled but it is very difficult. Here is an approach I have found that worked reasonably well.
Attrition - weaken the chosen-star(CS) as much as possible prior to combat. Dwellers is excellent since no save is allowed but I have found that repeated sallie templates will work based on mere volume of fire. A decent template hit will cause 6-7 wounds and hence kill 2 Chosen on average. A couple rounds of shooting from a pair of twin sallie packs can thus kill over a dozen chosen and you might even get lucky and have them fail a panic check!!
Pin & Assassinate - Tackle the depleted CS with a large block of hw&shield saurus arranged to keep steadfast while simultaneously hitting the flank with steggie (preferred) and scar vet cowboy. A cowboy tooled with cold one, dawnstone, dragonhelm, g/w & others tricksters shard will do nicely. With the chosen star pinned in place and unable to reform and a life slaan BSB nearby, the scar vet can kill off the characters one by one, earning some major points (WoC characters must issue challenges). Depending on how the chosen star is equipped it shouldn't really be doing too much damage to the saurus, especially if the block can be buffed by life magic. (most Tzeentch variants use h/w & shield and so are only dishing out S4 attacks.)
Grind & pray to the dice gods - Even with re-rolls, WoC are usually only LD 8 and thus can fail fear and/or stubborn checks. Consequently, killing the BSB also becomes very important as life buffs on the saurus will make repeated break tests almost a certainty. Between steggie stomps and the cowboy and occasional woound getting through from the saurus, the unit should be ground down over time or you may get lucky and break them.
Hazards - another favourite variant is the banner of rage (unending frenzy) and halberds, which makes the unit a lot more dangerous. Like any frenzied troops they can be led astray by a flank charge from a single file of skinks (who will undoubtedly lose combat and be pursued away from your lines). Alternatively, the same approach as above can be used but hit the front with the cowboy and then the flanks with saurus and steggie. The scar vet will be fighting challenges for several rounds and minimal attacks will be put on the saurus and steggie - you might even break even in wounds (but don't count on it). At the very least, the CS will be testing for losing combat every round since you will start combat up by +4). With luck, one of the break checks will fail and the unit can then be run down (now there is optimism!!).