I am a baddie. I play online and am a D racer and regularly finish 6-8th. I am a terribad. I got first at spa. If I can do it, I'm pretty sure most people can. I think my first tip is, don't follow the tips of good racers. They won't work for you if you are a terribad like me. My second tip, if you're a terribad, is to race the car you feel comfortable in, not the meta. The car you race well is the car you'll win in. Every tiny thing will be that much better in a car you can race well.
I did this on medium tires, pitted at the end of the 5th lap and got more medium tires and filled the tank completely up. I raced with default fuel setting (max power bby). I personally used the Mercedes because I find it to be very stable in corners and it just plays well to how I race. So basically, other than tires, I raced a very bog standard race.
The trick to this race if you're bad is to know where you can make up time, and then just take it easy the rest of the track. The first place to make a ton of time is Eau Rouge. I think the hard nosed racers say to take this flat out. I'm not that good. I lift as I hit the first apex then smoothly bring it back for the second and get back on the gas flat out for the rest of it all the way to Les Combes. You will make multiple seconds a lap up here. The second place to really make up time is Stavelot and Blanchimont. When coming out of Fagnes you want to lift slightly to ensure you hit the apex at Stavelot, and then full gas or mostly full gas, whatever keeps you on the track. When you hit Blanchimont lift until you hit the apex and then full gas again. I think here again the pro pro pros will say just go full gas the entire way, well I'm not that good and this guide is for baddies.
Outside of those two places, the only thing you need to do is brake early and make the turns. Take it easy. You might lose a tenth per turn but who cares. Your goal is to do just enough to win, not win by a mile. This is what I did and it worked for me. The weather is random, if you don't get lucky just restart immediately. I tried switching to inters and every time it dried up, where not switching it would just keep down pouring. It's so random it's not worth fighting, just restart and hope for better luck. For my winning run it sprinkled 3-4 laps but no heavy downpour, it was manageable. When it first starts to sprinkle you can perform the same strat, about a lap in you'll want to start braking earlier and taking it slightly slower. I guess one last tip is to try as hard as you can to stay on the racing line, this is a really fast track and being off the line is brutal, especially when it's wet.
I hope I can give hope to all the bads out there.