Isildur's Bane is able to respond to wounds taken in the Regroup Phase, or any phase for that matter.
Essentially, if you put the ring on due to an archery wound, he would take it off at the beginning of the regroup phase. If an Isengard Orc ability was sending a wound your way, he could respond again to put the ring back on and take wounds again. This goes for
the Ring of Rings as well. These two Rings are the ones you want if you are worried about taking wounds in the Regroup Phase.