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I only have some csm and 5 IK no Cerastus.

How do I beat a list that has 3 WK, 6 msu scat bikes, and 3 farseers?

easy cop out is to buy centurions with grav or bikers with grav.

What guns or melee weapons can I use in the CSM arsenal? What arsenal does IK have to fight WKs?

Heldrakes mow down scatbikes. MSU bikes are rougher though, since a drake needs a long life to pay back its value in 3man bike squads. IKs to kill WKs though? Only thing that comes to mind is to flood the field with Gallants and rush them. Be'lakor might be useful. Invisibility, fleshbane, etc. Not much else comes to mind.

Check out FW Chaos Knights. You really ought to convert one. Their chainsword gets +1 on the Destroyer table against MCs/GMCs and you can add a Mark (Daemon rule) for a 5++ save. Also, they have Dirge Casters to prevent D-Weapon overwatch.

Mark of Slaneesh in particular will reduce WKs to Init 1 about 28% of the time. Debuff a WK's leadership by one with Belakor, and that chance goes up to 45%. You can also tag WKs with Hallucination to ensure your Knights always get the chance to swing, and hit on 3's as a bonus.

Chaos Knights are LoW though, so you'd need run dual CAD. Cultists are probably best as they can prevent your Knights from getting charged. So basically -- 2x Daemon Knights, Belakor, 40 ObSec Cultists, and 350-450pts to drop into a 2nd HQ or whatever else you might need.

Tbh, playing an army based around 5 large expensive models, against an army that has range 36 str D weapons is always going to end badly.

Eldar have ranged superiority, and a decent chance of destroying a knight or even 2 every turn.

As others have said, be'lakor is a strong pick.
I wouldn't use him to go toe-to-toe with an uninjured WK anymore, due to stomps and increased durability, but his guaranteed invis and shrouded powers should help your IK survive longer.
You will need to use LoS blocking stuff to keep Be'lakor alive however, due to scatbikes.

you fill a land raider with cement and throw him at the wraithknight.. dont miss

Well, you've got a few things in the chaos arsenal which will upset a wraithknight.

A daemon prince with the Black Mace can make a mess - If I remember correctly, the Black Mace has fleshbane, which is still wound-on-2+, even against a gargantuan creature. It also has the daemon weapon bonus to attacks.

A winged prince of nurgle has a 2+ jink, so can probably evade wraithcannon fire on the way in, and (if it's a cannon armed knight) can probably job it in a turn or two. Plus, it's more mastery levels on the board to help keep your psychic tricks working and those farseers back in their box.

Be'lakor is also a good buy due to his suite of telepathy powers. Invisibility is good for protecting critical units, but Psychic Shriek is good for taking out small jetbike units because it ignores armour and cover, and guardians have only a so-so leadership value.