Fallout 4 difficulty question
If I weaken a legendary enemy on normal, then right before I kill it switch to survival; will I get a better legend drop?
No.
I haven't confirmed this through testing, but it's my understanding that enemy drops are determined at the time that the enemy is spawned. Therefore, changing your difficulty after the enemy spawns shouldn't affect the drops.
This article on farming legendary drops indicates that the drops are set once the dungeon spawns. This leads to the same conclusion.