Kaz backed up a bit, then tucked his cane under one arm and took two running steps before using the boost Tony made to half-jump. His arm made it up enough to grab onto the end of the staircase. Then he jammed his cane between two of the stair rails slats and used both hands and the impromptu handle his cane was supplying to pull himself the rest of the way up.
After he was up he laid flat on the remains of the staircase with one boot shoved between the banister slats to anchor him, and he held one end of his cane with the other being lowered for Tony to grab onto.

"Grab onto it, it's a lot sturdier than you'd think. I'll swing you as close to the broken banister as I can. You should be able to use that to climb the rest of the way up."