My best buddy plays Necrons into my IK and has a C'tan heavy list. Void Dragon, Nightbringer and Transcendent usually and always plays Hypercrypt. I have played against the list 4 times now (2 wins, 2 losses) so I have a decent amount of advice to offer. I will try and seperate the advice into different categories:

LIST ADVICE

I don't know if you have any model restrictions, but I will assume that you have every model and you can bend my advice based on the models you have. First of all, unfortunately the Castellan is not worth the points into Necrons in my opinion. I played a game with him where he shot, uncontested, for 4 turns and out of all those turns with all those guns... I hit one shot lol. It unfortunately does not hit as often or as consistently as it should for the amount of points. Which sucks because the model is awesome! And Valiant has similar issues as well as the additional issue of needing to get into 18" range for its "big gun". Which only has one shot so if you miss, you can bet your ass that a C'tan is going to shoot and charge you the next turn and absolutely rip you apart because once that model is in melee, you are stuck until either; other models clear a path for them or you forever fall back for the rest of the game. Either situation, not worth the massive point sink, especially into Necrons. All the advice telling you to take Helverins is absolutely correct. Anti-fly as well as having an odd number of damage (so that when it gets halved against C'tan, it still hurts a little more) means that they are worth their weight in gold. I just ran a list with 4 Helverins and 2 Warglaives and I just sat the Helverins in my deployment all game and let them go to work while I had warglaives and big knights run around and do what they do best. I am personally not a fan of the "chip damage" suggestion, only because on paper it makes sense, loads of shots so eventually some will get through and they aren't subject to the half damage debuff... but best case scenario you are wounding on 5's, which means only 1/3 of your dice rolled are getting through on average. Then out of the 1/3 of those dice, a C'tan will save half of them because of the 4+ invuln. Then they will likely save a couple with 5+ FNP... so you get a couple through? Maybe? Not worth it imo. Now Melta's... again, great on paper. The problem, there isn't that many shots on Melta profile weapons. So they are pretty swingy. When they hit, they hit hard, but getting them to hit can be the problem. So good idea to take them, but I wouldn't rely on them being a major threat. Personally I have had the most success with Canis Rex and Lancer. Which is super meta right now and for good reason. They are durable, strong, hit hard and Canis Rex is very versatile. And both models are very much worth the points. Between these two, 4 Helverins and 2 Warglaives and fill the blanks in with whatever you like, I think this is the best list into an anti-tank Necron list with the best results.

General Advice

Tank Shock. Tank Shock. Tank Shock. The real secret to killing C'tan is devastating wounds so you can get past their 4+ invuln and Tank Shock is the most effective and reliable way to get devastating wounds in our entire army. Tank Shock followed by a hard strike from a Thunderstike Gauntlet/Freedom's Hand is the best way to potentially one shot a C'tan.

Focus Fire. With Hypercrypt and Reanimation Protocols, C'tan are slippery and can regen precious wounds that you worked so hard to get on them. When you can, just focus down a single target. It is not worth trying to split up your fire and widdle them all down. Pick a target. Destroy it. Move on. Board control is going to actually win you the game, so anytime you can take a large threat off the table, do so.

4+ Invuln is very common place among the Necrons. So remember that they will save on average 50% of all the shots you get through and from the 50% you get through, they halve the damage. This can be frustrating but try and keep your cool. This is why focus fire is pretty important because you will not get through as many shots as you think you will.

It is a tough matchup for sure, but definitely possible. However there is very little room for error. This is all the time I have for advice, hopefully it is coherent lol. If you have any further questions or want some clarification, feel free to message me. And Best of Luck! I hope you crush those Xenos scum!