Sadly, DE hasn't done Defense targets well ever. But, it used to be tolerable when Crowd Control abilities could actually do their jobs. 20 hour Void Key defense missions were possible because of how old CC worked as the only way defense targets survive at high levels is that nothing hits them. But, that required CC to actually work (which has been getting worse and worse since the Eximus update) or you killing everything before the pinky toe of an enemy comes around the corner to see the target (this has been nerfed with LoS changes). The best way I can think of that defense targets can survive properly is that DE makes it so that each enemy hits defense targets with a specific damage value that doesn't change with level, kind of like damage attenuation but in a way it works for us instead of against. Also, even if the 2 above options were available still, you would have to get lucky with the terrible RNG loadout system. Good luck getting the few frames that can actually defend the target from ever getting hit.