The 20% magic reduction is from -Magic Barrier in DQ8-. not SH defensive buffs. SH defense buffs are generally notably better than that.
Shadow Hearts buffs are usually bad-ass on principle. Not to mention that, like I posted before, Rhapthorne's Magic Burst is slamming straight into Ricardo's resistance to Fire thanks to his innate element, and Rhapthorne doesn't really have much to recuperate the lost damage on him, before accounting for Ricardo's generally good S.DEF.
Just for Ricardo's resistances/weaknesses...
Assume for a moment that Rhapthorne uses Cold Breath -> Magic Burst. For Ricardo, this means he's taking an extra 32 damage from Cold Breath... Then drops the damage from Magic Burst by 67 points. Dhoulmagus can try to recoup this damage with Arrows->Kacrackle, but it's only 3 points more damage to Ricardo when everyone else is being spared an extra 10. If he just does Arrows->Belching Fire, Dhoulmagus is also losing another 15 points of damage off his best combo to Ricardo.
Note that Yuri2 can also easily take advantage of this by going to Fides his first turn instead of Neo Amon straight away for his first turn (and possibly his second, because a Galed Yuri2 using Energy Charge is FAST), while still having generally boosted stats from fusions.
EDIT: This has yet to mention that the bosses have yet do anything other than have trouble with killing a Ditto-D.Dhoulmagus through all those innate-resistances, 50% Ice and 25% Fire being the important ones.
EDIT2:
ALL TEAMS vs. Dungeon