Against WoC,
Lore of Metal is king
I have in two separate tournament games:
A) first turn searing doom of a unit of 8 chaos knight to remove look out sir then used golden hounds to remove the BsB
B) first turn boosted searing doom on 5 skullchrushers and a character, removed the skullchrushers completely leaving a lone character that later got broken on combat res
Sadly lore of metal is useless against daemons
If the WoC player is using a horde of mauraders or warriors, I like to throw in a scar vet on a cold one , use lore of light (also excellent against daemons) to keep him safe for a turn (light of battle) then cast phas, speed of light and time warp, they are hitting you on sixes, wounding on 4 or 5 and then after modifier you have a 3+ rerollable AS, then I'll hit back with 5 strength 7 (GW) on 3's with poison and a reroll wounding on 2's.
If the player makes a tactical error and comes straight for you, I have held up 50+ mauraders for the whole game with a single scar vet while my magic secured the win