Ah if its single target then yah probably hybrid. Breaths ignore shadows. Should be easy to test, could try with invincible or run sp2 and see if it still deals damage. Does liement/all for one absorb/reduce it?
I still think the best strategy is 5 sam 1 smn with perfect defense, giving the sams sv buffs and cor rolls before entering. He usually dies in VD with that setup in under 30 seconds but may be impractical for most people. After PD, SMN switches to carbuncle and activates astral conduit restoring their MP and spamming healing ruby incase sams need cures(sams use Yaegsumi at start too).
OfA does help mitigate it, it's basically a Phalanx for magic. Liement can absorb as well, though I'm not sure if it absorbs all of it, hence my feeling that it's hybrid.
I have seen a PLD with Invincible up take damage from Light Blade (it wasn't much, they had Aegis). I never tried using Sforzo, guess I could? Then again...
...I agree with you regarding the easiest way to do it- our group generally takes in 4x DD GEO SMN with outside BRDs and a COR or two, depending on who's bored in the LS.
GEO is ridiculously good for this, plus if something goes seriously wrong at least you have someone who can nuke fetters.
Edit: I think SAM should save Yaegasumi for when PD wears, if it isn't dead yet. Assuming your CORs did their job pre-entry, all DDs should be entering with 3k TP anyway so using Meikyo isn't strictly necessary.