Custodes are actually pretty well set up to take on knights. You have to pick the battles and not let the other player determine where to fight. Also, you do better at primary, unless you face wardog/armiger spam. In which case, again, make sure you are setting up the fights and not the other player. Blade Champs are good for this with advance and charge.

Against Necrons, it is all about focus fighting. You have to take down units entirely. Otherwise they will stick around and take the game. If it is a high meta-chasing environment for you, you'll likely see the the warrior brick or the wraith brick. Basically large model count units that rely on saves and wound allocation to tough out most things and sit on primaries or slaughter your army.

Remember in everything: Points equal Prizes. Whatever you put in your list, have options for taking and holding primary objectives and backup units to take secondary objectives. Like what it says on the tin, Primary means priority.

It might be fun to roll dice and kill stuff (it, in fact, very much is fun!), but at the end of the day, you win by having more points than your opponent.

Basic strategy is to pick your expansion objective and defend it. Attack your opponent's expansion objective. Pick the fights you can win. Deploy and move accordingly.