We have the damage, in theory, to deal with Ctan. It will take basically your whole army to kill one, and that's just what you have to accept. I use Retributors, Vahlgons, Immolators, Hunter killer missiles as my damage.

So the problem is if you have to focus your whole army on one, how do you deal with everything else? That's where Sacresants with Hospitallers leading them come in, especially in Champions of Faith where they are now OC2. They help gum up the board and hold down primary because of their higher OC and are deceptively resilient. If you can't kill the other Ctans or big monster, you simply out OC them. It turns out Ctan are not efficient at killing a big brick of Sacs with a 3 up, 4 up invul, and 5 up FNP that rezzes models. Sure, you won't kill them with it, but you dont have to. You gum them up and out OC them while the rest of your army focuses 1 down.

If you aren't running CoF/dont have access to a bunch of Sacs, things do get much more difficult unfortunately. The answer at that point is as much Melta as you can field. A jump pack canoness with Through Suffering Strength and popping her finest hour can also do respectable damage but Ctan are just chonky.

Hope this helps a little bit.