I... don't think you know where defense goes?

So the formula for damage against you as given on the wiki is currently:
{Enemy_Attack * (1 + atk_buff - atk_debuff) * 'weak point correction' * (1 - damage_cut_total)} / {10 * (1 + defense_bonus) * (1 + def_buff - def_debuff + assist)}

'weak point correction' = element resistance/weakness. Specifically, it's the 1.45x if you're weak, 0.75x if you're resistant.
The way I worded things in the previous post was a bit off. Reflects actually stack with damage cuts additively, hence 'damage_cut_total. Cao Cao's page on the wiki specifically cites her reflect stacking with Raikou's damage cut for a total of 70% damage reduction. Element resistance buffs apparently also get added to damage_cut_total.
defense_bonus = what your soul's been picking up from mastering certain lines
I'm not sure what the 10 is doing there, unless the internal enemy attack values really are just that much bigger than what we players see in game play. For our purposes, we can ignore the 10.

So basically, when you're going from no defense buffs/debuffs to +30%, you're dividing damage by 1.3 instead of 1. That's functionally the same as multiplying damage by 77%. 30% is also really freaking big as party-wide defense buffs go. Andromeda's def buff for another example is 20%; that'd reduce damage to 1/1.2 = 83.3%.

An example to put it all together...

Let x = enemy attack/base damage
You land -50% atk debuff (or more, but let's say we're in the world where the 50% debuff cap is in place). Enemy has no atk buff.
Damage is now 0.5x
Your element is strong against the enemy; that's another 75% multiplier, bringing damage to 0.375x
Your main eidolon has 10% resist against the enemy's element (Jack Frost versus Fire for example). As far as I understand, this actually goes to damage_cut_total. Let's add a 30% damage cut to that (Snow Raphael for example); damage_cut_total comes out to 40%. 0.375x * 0.6 = 0.225x
I'll ignore how much +defense your soul has to simplify things; also it's not like your kamihime benefit from that.
Let's say you throw on a +20% A-frame def buff (from Andromeda) and a +12% B-frame def bufff (from... Gabriel). That adds up to 32% for defense buffs. But, Snow Raphael and Gabriel have +defense assists. I don't know how large those are. I'm assuming 5% each, as the +attack assists I've seen are listed as 5%. So for those two, their defense buff total comes out to 37%
0.225x / 1.32 = 0.17x
0.225x / 1.37 = 0.164x
So damage has been reduced to just a bit over 1/6th of what it originally was. For Snow Raphael and Gabriel, just under 1/6th. And in practice, your soul probably has built up some +defense from at least the Joan of Arc line by now.

...looking back on damage cut stacking, I think Cupid suddenly just jumped up a notch here. If you throw Cupid into the example above, you can add on her 30% fire resistance buff (ok, at this rate we're basically running something like Andromeda/Ryu-Oh/Snow Raphael/Gabriel/Cupid for demonstration purposes). Damage_cut_total would change from 40% to 70%.
0.375x * 0.3 = 0.1125x
Keep the same def buffs; 0.1125x / 1.32 = 0.085. That's in between 1/11th and 1/12th the original damage.
Not that you'd actually run such a party (unless that's what you have with nothing better), but the example gets the process across I hope.