Buy a GOOD 1 and 1/2 inch wood chisel and chisel the top of the sringers to level with each other. After doing this you may need to raise the hight of each stringer flat (not angled) shims to keep the hight of the step even with the other steps even.
Better than buying a chisel, buy a 4 1/2" angle grinder, rubber backer pad, and some 36 grit discs. This tool is like my right arm. I use it for leveling, scribing, coping, etc, etc. In no time flat you can grind to a line you draw on the high stringer. Don't use leveler mix. If a stringer is low, squirt on construction adhesive and embed a shim in the glue, then add more glue on top of that. Stringers are notorious for shrinking after they are cut, leaving the flat area for the treads leaning downhill to the front and the riser leaning to the back at the top. That is the only slack I give a framer. I have spent hours trying to get a set of stringers into specs after a framer has butchered making and installing them. One of my BIG pet peeves!