Supposing you had no way to back fill the low side, your best course of action would be to "terrace" the incline. Over the distances you provided, you could obtain quite a beautiful and variegated multi-level garden. Not all terraces would have to be overplanted or flowered, grass would be a pleasant divider with cut back grade transitting. (left to right drop or vice versa) The types of fill plants depend on your locale. Terrace can be achieved by various means of wall to earth tie in. Example: railroad tie or 6x6 treated stock with 3/4" rebar staking @ 5ft min intervals with 4-6 foot insertion depths. And then install an occasional back beam inserted 4-5 feet perpendicular to the original grade face. Myself, I am picturing a multi-tiered japanese garden with bonsai, grey rock, and juniper, with the appropriate colored perennials. Bill