I started playing King Arthur the same hour he was released. I was insanely hyped for this god and I've probably played him the more than I've ever played anyone in such a short timeframe. I think the amount I've played at once qualifies me to give advice so here I am. I think he takes a while to get the hang of but once you do, it doesn't really matter what you build. I'd say so long as it's tanky, you're going to be fine. In fact you'll stomp mid game. I find that before his gladiator's shield is online you need to play slightly safer, but once you have it unleash hell. I main joust and almost every game between levels 6-12 I run into all 3 enemies, alone and come out alive with at least 1 kill. He is amazing at applying constant pressure and I find it easy to be top everything despite, even damage, despite the tanky build. Unfortunately I find he falls off a bit in the very late game. With my team we can dive whatever at levels 13-18 however the glorious 1v3's are unlikely unless you're ahead by a lot. If you're in full lategame, with everyone fully built and lvl 20 you need to actually think about when going in. This doesn't mean he's bad, if you can coordinate your attacks he will still do amazing, you just have to actually get that coordination. I mostly play solo or in a party of 2 so it's not easy to get that, hence the feeling that he's not so great lategame.

To summarise: Wait until glad shield, play stupidly aggressive, calm yourself at the end and play smart with your team