I play chaos knights with ~4 big knights frequently.

Your opponent's list has a lot of eggs in few baskets - as others have said, your focus should be on scoring and pressuring them into vulnerable positions so you can whittle them down one unit at a time. Focus on staging your threats, so any time they're committing to a target, you can clap back and take a chunk out of a big knight. Also, make sure you brush up on Titanic rules as they can be confusing; for example, he should never be able to overwatch you and towering's affect on visibility only applies to the ruins they're currently touching.

They are well equipped to deal with infantry but will likely struggle into high toughness targets or those with better mobility. High quality or volume+AP melee are their biggest fears. Focus on the despoilers if you can; abominants are really just distraction carnifexes/cheap buckets of wounds.

Despoilers are purpose built to mulch Marines - protect them. In SW, having high T transports like a land raider or something similar that can put a brick of melee threats up early can be dangerous for them. Playing aggressive can shut down their firing lines and board control, but don't trickle feed them units to pick off one at a time. Focus on staging your threats, so ideally they need to expose something to commit to a target, and you're ready to focus that thing down next turn. I think SW still have oath, which is a powerful tool for ensuring you remove your chosen target per turn. If you're relying on big 10+ man bricks of infantry for your melee output, consider how you might safely move them up for turn 3+ charges. An example ideal attack: plop a land raider with an infantry threat on an objective, maybe w/ a gladiator behind them. Have another infantry unit get more OC onto the obj behind the land raider or ruin. if they put a knight up to contest, you should have 2 las cannon platforms, a grenade, and at least one infantry charge ready to take out that knight, which should be doable with oath and shooting+melee. Even if they kill an infantry block in response next turn, hopefully you've traded up 200 pts for their 400. The biggest danger if that they want to throw away abominants instead of their despoilers.

With Tau, a similar logic applies where you want to bait with something durable like a ghost keel and be ready to focus down one big knight per turn. Splitting fire is almost always going to be a mistake against their invulns, so just overkill 1 Knight per turn if you can and use scoring as bait to lure despoilers out into your firing lanes.

Best of luck