If you start poking at that its going to be huge hole, the problem is it is rusting behind it too as you see at the seam. It can be repaired but to do it right its going to get expensive. I would start by removing it about 1/3 the way down by cutting and drilling out the spot welds then see whats the rest looks like inside.
The whole rocker and maybe some internal pieces may need to go, this is getting into big $$$, the cheap way grind it down and patch over it but that's a loser right from the get go, the bugger will be rusting before the paint dries (exaggeration). If you can weld and have the time and patience it isn't that much money in materials.