Remember the scenario I was going to write up on One Page for the contest this year? Well, here it is; click to enlarge. It fits pretty nicely.
Knowledge is a crucial resource in surviving this challenge, as is having the right anti-devil equipment (silver weapons, holy water.) Defending the penitent is job number one; there should be enough party members to surround him/her and any party members who do not fancy being in the front line. I was relatively merciful to the 4th level party that tried this, and gave them a nearby order of holy knights to get supplies and clues from.
A high level penitent is a mixed blessing; they can resist attack and saving throws better, but let more devils through the barrier.
Let me know your thoughts and comments before I submit this!