In a normal game, the Imperial can focus fire on one character and by doing about 10-12 damage Wound them. They cannot ever heal back past that threshold and they rarely "double heal" in a turn since many of the points would typically be wasted (because if they ave 10 points to heal, then they are probably passed the threshold already).
With Legendary rebels, the Imperial player has to do 20+ damage before the victim activates and heals. Furthermore, the rebel can heal 10+ damage on their activation and all of it will be typically be used (because it is not uncommon to be in the 10-20 damage range and there is no barrier).
This makes it near impossible to wound such a character if they are set against it (as they will be in several very important missions).
My proposed solution for this is to insert a healing threshold equal to their non-Legendary Health that has no game effect except that, once crossed, the Rebel will always have at least that much damage.
So if a Legendary Rebel (with 10 Health initially upped to 20 for Legendary) takes 15 points of damage, they would not be Wounded, but they could not get any healthier than 10 damage against them no matter how much they heal. This means that another round of 10+ damage will actually wound them - at which point the card is flipped etc. as normal. A similar threshold would exist between Wounded and Defeated.
I think that this simulates very well the "normal" situation with 4 Rebel characters - the situation around which the game was built, tested, and balanced. Without this change, some missions are just about impossible for the Imperial to win (and these are key missions). It is certainly WAY more difficult to wound one Legendary character than it is to wound two normal rebels, and that seriously unbalances the game.
Please note that Quad movement and being unable to be in 4 places at once are different issues entirely, as are any discussions about time constraints and other mission-specific factors.