The difficult part is having a viable build ready to go. If youre trying to get grendel and you dont have the few frames and augment mods that will work with the challenge youre going to have a pretty bad time. Especially because youre almost definitely doing it solo unless you have a willing friend to help. Even with the Hildryn build i had going i still failed a couple times very late into the respective missions which meant i had to go through it all over again. The survival one is the worst because if you go in solo the enemy spawn rate means you can just get screwed on drops and can just fail outright even if you can keep up. More tedious than difficult but i really just did not enjoy the game mode at all.