Hmmm. On my cars, the parking brake was adequate to lock the wheels to torque the inner and outer axle nuts... otherwise yes, you could have someone stomp on the brake pedal, preferably with engine running to provide vacuum & therefore power assist! The outer axle nut should be done with the tire mounted & wheels on the ground, that makes it much easier - leave the wheel center cap off to access the nut. For the inner axle bolts you cleaned the threads with a die, applied blue Loc-Tite, and oiled the contact surface - right? 100Nm isn't a lot, you should be able to do that, but I understand it's more difficult with the car on jackstands instead of a lift. It's a breeze when you can stand under the car!
