Not as hard as it looks, I even got better results than on usual driving with aids on, just couple of tips.
1. You can do it in any race within RX championship in free play tab so pick you favourite track and skip races till you reach it, for me Hell, Norway is easiest.
2. You can drive all 5 pre races on any settings you like, just switch all aids off for final 6th race (manual sequential gearbox is enough), set AI to 100 and change camera to cockpit (any of 3 cockpit cameras will do).
3. I used Audi S1 with settings from below video, just tweaked all settings from Diff and Damping 1 click less than on video and final Gearing on 200.
Hope this helps
It looks you may need drive with 100 difficulty all 6 races, some people report changing settings just for last race doesn't work.