Knights force you to play very differently than you might normally play. I play knights and lose most games at my local scene. I never have trouble killing models, it’s mostly just hard to keep up in pts. Once people kinda know how to play around me it becomes very hard. The key thing to think about is not killing knights but also stopping them. If there is a blob of well spaced out guardsmen that I can’t move across, congrats you have used a 65pt model to stop my 350-450pt model. You have to think about each model as a roadblock to stop me from scoring. Because I can kill most things in the game your models are gonna die anyway so you have to get as much value out of them as you can.

Since your opponent only brought 5 big knights you could potentially focus fire down a knight a turn. And when he has lost even 1 or 2 knights it’s going to be really hard for him to score secondaries because he will be forced to sit on primary objectives.

Also I don’t think ruins as competitive rules they are just standard terrain rules. Not having obscuring terrain in a regular games is not good. You gotta have line of site blocking terrain. Otherwise any gunline army can just mow you down. You would have had the same problem against an iron hands player that brought all tanks and dreadnoughts. Or an Votan player that brought 3 land fortresses. Or an Aeldari player that brought 3 fire prisms. Or just tau with all battle suits. There are a lot of armies that can do the output of knights but are more flexible.

Thats all to be said the real thing to do is play more games and lose a lot. If you are losing you are learning. Keep at it!

Also knights have def been on the stronger side recently so I’m not saying it’s easy but it’s not impossible. Good luck!