I will assume that the problem is her size relative to you and her strong muscletone. First, stay away from a soft bed - use extra firm or the floor. Missionary is the probably best with her legs 90+ degrees up, knees bent, and pushed out. She will be positioned to make it easiest for you to enter. Another good position is missionary with her butt inched all the way up to the corner of the bed with legs up. You meanwhile are flatfooted on the floor - you grab her by the knees and you are in control.