...Last game 5+oldblood held a horde of 40 tzeench marauders for 5 phases...
40 marauders (im assuming HW and shield, full command and MoT) cost 280 points
5 CoC (full command) and an old blood (assuming 100p magic items) cost around 500 points
YOU didnt hold HIM for 5 turns. HE held YOU for 5 turns!
this is the weakness of CoC. they are good, but they are also very expensive for what they do. for 500 points you could get 43 saurus warriors. do you think that 40 marauders would last 5 turns against 43 saurus warriors? no. for 280 points you could get 23 saurus warriors. they would likely have dispensed the 40 marauders faster than 5 turns too.
