So I noticed when doing my fights that Morgana was innately immune to fire damage with 75% resist parameters, which shouldn't be the case since (-20% 75%) < 100.

Things I've discovered (tested with 10000 needles):

1.) Elemental resist parameters ignore base Morgana's resists, instead they replace them entirely, so Morgana will have 'equal' resist to the parameters instead of adding resist to her base resist. This means you can use earth/water/ice/lightning damagers for the most part.

2.) Elemental resists are not correct. On 30% bonus resist, she actually has 39% resist (30 * 1.3). On 75% bonus resist, she actually has 131% resist (75 * 1.75). You might be wondering how people are still oneshotting her right? Well, somehow, imperils are being amplified by the same amount. So a 135% imperil on 30% resist challenge is actually 175% and on 75% resist challenge, it's actually a 236% imperil!!!!

Sadly I've run out of orbs so I don't know how 30% 75% works. Will update if I find out, or if someone else does.

Edit: 30% 75% does not act like the other 2. I believe I've found that it amplifies Morgana's resist by 45% (so she has 152% resist) and amplifies imperils by roughly 204%.

Edit 2: If Libra is correct, then for some reason 30 75 makes her resist equal to 75%, which makes the formula work since it amplifies both resist and imperils by 205%.

Added video. 10000 * ((1-.75 * 2.05) 1 * 2.05)=15100 (decimals rounded down in ingame formula) and 10000 * ((1-.75 * 2.05) 1.35 * 2.05)=23400.