Ground level deck - how to attach bearers to concrete
If possible, I'd put posts at the ground level side, even if you have to dig out a little.  Even if it's just a few inches below the beam, it's easier for the very reason you mention.  It will keep the beam drier.  If the post ever rots, it's a lot easier to replace the post than the beam.  You may be able to make the posts integral with rail posts, too.