Tension it maximum with puller over night.
Tension it to maximum etc.
If you brought it in to a mechanic to work on, they’d just use a big enough and strong enough puller to break it free in one go.
I removed the steering gearbox with pitman arm attached and wrestled with removing the pitman arm on my tailgate where I could get a could purchase on it.
In my case, it wasn’t a monumental struggle but it was on there pretty tight.
You might try the two hammer method if you can get a swing on it. Basically use two hammers of the same size and hit the outside of the arm where it sits on the shaft from opposite sides at the same time. This method often helps to break tapered shaft connections free. The pullers have to overcome the greatest force. Alternatively you can put the puller on and get it under tension and then do the two hammer trick.