The FIA World RallyX Championship unlocks after you get promoted to the Professional level in the regular RallyX Championship. The difference between the two is that one season lasts longer since it has more events and the AI features names from actual RallyX drivers. Other than that, the difficulty of this league is somewhere in the Professional level, but AI mistakes are not very rare, as they will sometimes spin out coming out of banked corners or crash into you like they didn't even see you on the track.
Given the difficulty and the... "temperament" of the AI, you need to make sure your car is fully upgraded. A good choice would be the 208, as it is the least demanding car in the Supercar category. I personally had no issues with the DS3 and would suggest trying it out in a custom championship. Once you have all upgrades installed, it's crucial that you do well in the qualifying.
That's because besides getting you through to the knock-out stages of any event, they also dictate the starting grid. Drivers with more points/better times have the benefit of starting the subsequent races from the front row.
Since the difficulty is slightly harder than previously, don't hesitate to keep restarting if you make mistakes or if the AI thinks you're playing Destruction Derby. After a while you'll find your braking points and the right gears for every corner and you'd be able to be consistent throughout the championship.
One last piece of advice that most people don't pay much attention to, would be to take the Joker lap at the last chance, since you want to keep the other drivers in a distance if you're leading the race. Respectively, if you don't have a good start, complete the Joker lap asap just to get it out of the way and focus on the rest of the track.