The Agony Aunt?
As has been said, arm pump comes on a lot quicker riding natural, mainly because you rely heavily on using your brakes to keep you staying put. So try and ride using your brakes only when you need to. Instead of holding on your brakes to stop you from rolling backwards, keep the pressure on your leading foot just enough that you don’t roll backwards/forwards.
It also might help if there is another rock to lean/wedge your front wheel against to stop you from going anywhere.
Being "good on two wheels" also counts for a lot. Being able to hop around on rocks on the back wheel is fine enough, but when the situation comes where you can’t do that, you need to be able to know exactly where both your contact points are for your tyres. For example, if you pedal kick forwards to place your front wheel on a rock, you need to make sure you know where your back wheel is going to land at the same time. Situational awareness is key.
Going back to what I was saying about using your brakes. Try to use either one or the other. If you're on the back wheel, there’s no need for grabbing a load of front brake - building up arm pump for no reason. Same goes when you're going through a section on two wheels - often there’s no need to use the back brake, conserving your energy in that arm for when you really need it.