This hill is steep, so I have a hard time climbing it.
This hill has a gentle slope, so it's easy to climb it.
この場合、次のような言い方ができますよ。
ーThis hill is steep, so I have a hard time getting up it.
「この坂は急なので、登るのに苦労する」
steep で「急な」
ーThis hill has a gentle slope, so it's easy to go up.
「この坂は緩やかなので登るのが楽だ」
a gentle slope で「緩斜面」
ご参考まで!