Getting an A* requires you to know everything well and be familiar with the questions, physics and maths tutor as mentioned is very helpful, but you really shouldn't be thinking of the bare minimum as question types may change, your performance can vary etc, so aiming for the precise boundary and stopping once you barely score A* in a past paper is very risky and there's a chance you'll narrowly miss the A* on the day, so if you really want the A* you should target a more secure mark