1 Perfect at landing on the flat in nil wind
2 Ability to fly a fast final with steep approach to the roundout - need to develop this step progressively or you'll hurt yourself badly
3 What next? Crosswind? Slope landing?
OK, I'm not claiming to be a FOTL expert, but we do plenty of hillside landings at Diablo. Hillside landings (I'm not talking about a gentle uphill) should be a skill every pilot has in their arsenal.
Frankly, hillside landings are easier that flat slope nil wind landings - even the steeper ones. I don't thinks you need to be a "perfect" flat slope lander to work on hillside landings. It may not be a novice skill to work on, but you don't have to be a H4 for example.
Starting out with a somewhat gentle slope is best - but not too gentle. Learning to come in with good speed is important, although it doesn't take blazing speed to do most uphills. It's really about recognizing that the flare window is going to open much faster than normal, and being able to feel your glider telling you it's time to flare despite any groundpeed signals. On steeper hillsides, the flare may happen just as you get close to the ground - virtually no ground effect.
One of the biggest pitfalls to avoid on hillside landings is the temptation to fly into the wind, but cross-slope. Don't do it. Many times, the prevaiing breeze (even a gentle one) will wrap around a hillside. A new-to-hillsides pilot will succumb to earlier instincts and try to land into the wind - bad idea. A decent hillside slope will negate alot of breeze, and attemping a cross-slope landing is a recipe for disaster.
Once you've gotten the feel for gentle and moderate uphill slopes, you'll know how to estimate your approach for steeper slopes, crosswind and downwind landings. It's actually pretty easy to learn this skill; most pilots pick it up very quickly.