I assume it comes in at least 4 pieces, I would assemble the top 3 sections and then raise it against the tree. Then with the help of a friend try lifting and angling the top sections up and against the tree while sliding the last section in place.
Or assemble the whole thing and then tie a rope to the top section. Have the taller of you lift the top portion while you keep the legs from sliding or lifting at the bottom. Once he has it as high as he can get it, pull on the rope and walk away from it in the direction of the tree to be mounted against, at that point the weight should keep the legs planted as the weight transfers down into the ground.
That's why I don't use them much and when I do they usually stay put for years at a time.