A $20 four foot level will work just fine. That and a STRAIGHT 8' 2x4...
Yes, and a chalk line. Mark one point on the house, then another point on the house 8ft away with your level and 2X4, then snap a chalk line through the two points past the 15 ft mark.
To the OP
Set the tops of the 2X4 at the chalk line and use the level to set the 2X4s. If you want a slope use a piece of 1X (3/4 inch) n the "low" end of the level and you'll have 1-1/2 in slope on 8ft.
I used a water level (clear tubing) on the last house footer I did, it works fine but you don't need that for what you're doing.
BTW present day 2*4s 1-1/2 X 3-1/2 inches.