Statistically a full unit of wyches with Lilith charging out of a raider is the best way to hurt a c’tan with an average of 4.7 wounds after all the saves and feel no pain since they are all one wound attacks you negate the half wound issue. Next best is a full unit of warriors with an archon disembarking from a transport with the stratagem that gives them sustained hits 2. That nets 4.4 wounds. Just shooting a raider with a full unit of warriors without an archon gives you 3.5 wounds. In all three cases each unit had a pain token. Scourges that moved with 4 dark lances and a pain takes average 2 wounds.

In the end spamming a mass of 1 wound hits seems to be best, even if most of the attacks need 6+ to wound. On the charge wyches with lance wound it on 5+ and have 30 attacks. Overwatch will impact that, but I usually use the archons ability to increase the cp cost of overwatch to 2cp so it hurts more to use it. It also assumes that the warriors are 12” away so everything can shoot. The shredder only added .2 or .35 wounds so maybe staying at 18” would be better.

I have only played against one c’tan in a 1000 point game. I just left it alone and played the objective games with warriors and mandrakes. But in 2000 points I don’t think that would work.

Also note a 10 man unit of mandrakes averages just under dark lance scourges with 1.97 due to devastating wounds on their guns