I'm curious about this, too. Griffon with all relevant GE's gives 90% damage debuff and 225% HP buff, leading to tank crusaders having near-invincibility. Not that an immortal tank really has any purpose other than pushing one or two more areas, but still, it makes for an interesting topic figuring out how a "120% damage debuff" works out. My first thought was making them multiplicative, so that if there is a 90% debuff and a 50% debuff, the affected crusader would take 5% damage, but if a SINGLE buff amounts to 120%, then...that mucks the math up a bit. Like "dividing by zero" a bit. I highly doubt the affected crusader would be healing 20% damage otherwise dealt.

Since it's capped now, is damage reduction from multiple sources going to be additive, rather than multiplicative?

There was actually a glitch a few months ago where the frontrunners were ACTUALLY immortal. Not "They never died" so much as they never took any damage at all. It didn't matter what crusader was in front. It was literally unkillable. Ask anyone (that isn't a troll) - it was the opposite of useful. Made it abundantly clear that bosses were mostly unaffected. Even with AoE abilities like Merci's and Petras, the amount of health enemies have is far too much to make a reasonable difference. Merci's ability caps at 100% DPS, so even if you stack 3000 enemies, you'll only be dealing 100% DPS to all enemies.

I used a demigod tank in order to push up to area 1000, but not without some major mechanics abuse in the last few areas, as even a near-immortal tank will cripple once enemy DPS catches up with your formation. I had Littlefoot being buffed by a GE-affected Dark Griffon. That's baseHPx1.5x3.25 and taking 10% damage. At level 6000, that's 31.2k HP, and effectively multiplied by 10 considering the defense buff. So it was able to resist just fine. Even at area 980, enemies were still no major offensive threat. However, they WERE significantly more difficult to kill, allowing them to more easily stack up and layer on the individual DPS'. Merci will help crowd control enough to push maybe a full 5-block chunk if you use Savage Strikes to buff damage near/at a boss, but overall, that horrid amount of defense doesn't amount to anything useful in terms of ruby gain. It is more of a novelty, for use in specific challenges. If you're just going for casual ruby runs, best to focus on DPS with your tank just doing whatever it can by itself. Most tanks get their job done well enough for casual runs. Save the "UNKILLABLE!!!" setups for annoying objectives that make them necessary. Otherwise you're just wasting time an effort better spent resetting and starting a new run.

Just as a small reference point, when I switch from my gold-find build to my DPS build at the end of a run, even with Merci having his GE making his Deflect Evil skill stack in increments of 6.25% and Mindy copying 75% effectiveness of two major DPS-buffers, I typically only push another 5-10 areas, which makes minimal difference. Though, I don't know what your average area clear is, so for all I know, it might make a bigger difference for your current progress in the game. I'm purely speaking from the perspective of someone that reaches 900+ with every run, so I could possibly be heavily biased and out of touch with the people still picking up speed (with all due respect).

Yep. Left the game running when I went out, it crashed on a boss at 315. Restarting the game, so the patch is applied-- no exploit for me. :)