Which situation will cause more damage to the opponent, example: Attack first with Nordri, lowering defense by -54% and then attack with C-Ulmer. Or the other way around: Attack first with C-Ulmer giving Nordri 48% more attack?
Defense down is better than attack up.
How about doing def down first, then elemental defense down. They stack and will give you the bigger damage per tile or special.
Well it is a rather complicated calculation there. Because more factors than just those 2 numbers are important.
If the only thing you compare is the EDD vs AttackUp then def down is more powerful in increasing damage, while also having the bigger number. But also the size of the hit matters and Nordris hit is bigger than Ulmers so it may be beneficial to fire Ulmer first to increase the bigger hit while Nordri first would improve the weaker hit. Often you have more than just two heroes like during events, when you have CGunnar, CUlmer and Nordri all 3 increasing the damage of the final hitter. CGunnar has to be fired first of course, but I believe most people fire Nordri next in that scenario, but I wouldn´t bet my life on it actually being better, but I don´t think the difference will be huge either way.
It’s exactly that question, in cold numbers, Def is better, but as you said: Nordri’s EDD is much bigger than CUlmer.