Any debuff upkeep shouldn’t be a problem if you bring a Pardoner to Discern Evil extend the debuffs on the boss, unless someone decides to overwrite.
All missile attacks are Missile + Pierce, except Schwarzer Reiter Concentrated Fire, Caracole, Limacon, and Retreat Shot. Those are Missile + Strike. I haven’t seen Missile + Slash yet.
Since you guys WB a lot, I wonder if they removed the Decay effect being applicable on Marnox. Decay HP decrease + extra missile damage was very interesting.
Edit: Vs a flying boss, have a Hunter use Snatching with the attribute on it. It will drop to the ground and receive +100% Slash and Strike damage.
Edit2: even on non-flying bosses, have a rogue capture Psychokino Raise. Then use it on the boss since captured raise works on bosses. Apply snatching.