Looks as though you're doing the easier thing in using mortar-less block. Lowes, Home Depot and others generally sell some varieties of these now. You only need to level them front-back and really not that critical, as most of them slope backwards anyway the way they join together. They can undulate side-to-side.
There are some very high mortarless walls around here. Think the highest (in tiers each about 6-8') is the one on the north side of the Lowes site north up 25 from Charlottesville.
I built a long wall in Baton Rouge and another just out my window at the base of the slope NE of the pool here. I built a small one on top of a pair of those ATV ramps placed side by side to keep my driveway from caving in on the inlet end to a culvert.
Ralph