How strong is this magic arrow anyway? Will the amp damage suffice against the bulk of the phys immunes out there?
Your Amp should help, but you can also kill them with WWS's in-built Level 20 Magic Arrow. I'd also recommend equipping your Merc with Crushing Blow items or having 20-30% Crushing Blow yourself (so long you don't compromise your setup too much). Once you break the PI on the monster, CB can whittle it down very quickly and make your life a lot easier. If you put something like a Hone Sundan Yari (45% Crushing Blow--eth upped Hone Sundan Yari is a beast too), "Obedience" Thresher (40% CB, *massive* damage, other godly stats), Guillaume's Face, etc. on your Merc, he should be able to turn PIs into sushi in about 2-3 seconds.
Another thing you can try is having a "Harmony" Bow on switch to pound the PI with elemental damage. "Harmony" comes with a whopping amount of elemental damage. With Strafe, this damage adds up quick. Still, I think the best option is to load yourself and Merc w/ CB.