Someone here may have more knowledge and experience than I do, so i welcome comments...
But this is what I do -
I take a deep cycle RV battery with two sets of posts. I connect one set of posts to the battery charger. I use jumper cables and connect the other set of posts to the "bad" AGM battery. I set the battery charger to charge. I have a battery charger that starts charging around 20amps, and gradually slows the rate of charging down to a float rate. I leave it for a week or two.
If the charger has decided that the batteries are charged and has dropped to the float rate, I try to connect the AGM battery by itself and run the reconditioning function. Frequently after couple of days I'll get an error message and the whole process starts over with connecting it back with the RV battery. But after an iteration or four, it frequently brings a battery back to life.
I have a couple of old optima redtops that are now showing 13.4V after doing this. They started the process with the charger unable to even see them.
Clint probably has the best and least aggravating approach. But I'm cheap and hate to dispose of anything that still has life in it. And as long as one of my MVs will start, slave cables will start the rest, so individual reliability isn't critical for me.