Wear gloves to save your skin, especially your knuckles.
If you are scared of getting under the vehicle or do not like getting greasy and dirty, have a drink before you get started. Do not drink too much because working on heavy vehicles can be dangerous and so you need your wits about you.
When working under a vehicle, use redundant lifting/holding devices. You do not want to rely on a single piece of Chinese-made equipment to protect your face from the vehicle falling on it. If lifting the rear of a vehicle, let the vehicle rest on jack stands at the jack point by each rear wheel and then place the floor jack under the differential, jacked just enough to apply a little pressure but not take the load off the jack stands. If lifting the front of a vehicle, double up on the jack stands, placing two at each jack point. If using ramps, wedge jack stands at jack points on both sides of the vehicle after the vehicle is on the ramps.
If a nut or bolt will not come off with an air impact wrench (or impact driver), do not switch to a breaker bar because you will only shear the bolt or stud. Instead go for the nut breaker, which will sacrifice your nut in favor of the stud or bolt.
If a nut or bolt will not come loose, try spraying it liberally with penetrating oil and allowing some time to wick into the threads. Then hit it hard with a smaller socket that barely fits over the bolt or stud.