Only 2 of the 4 1 vs 1 legends can be beaten by blood mage (I don't count the legendary dragoniod since this is a 2 vs 2 boss and thanks to the changes the staff made to make the AI focus on one player at a time instead of attacking an opponent randomly they will only focus on one person at a time until they kill them meaning that this boss is an even easier and even more of a joke than ever) and those are Legendary god of war, and legendary Matrix and here is why.
1) Their side arm, primary and aux all do the same damage in other words either energy or physical only.
This makes so a blood mage doesn't have to use energy to add shields and can focus on holding energy for healing seeing as in the end game Blood mage will only get small amount of energy from the bosses once their energy is nearly if completely drained.
2) The boss has reroute
this means the blood mage control the bosses energy so they can loop energy parasite to keep getting the needed energy for healing.
the other two bosses legendary titan and dage can't be beaten by blood mage because due to the nature of the ai having both physical and energy damage with their strike primary and aux (for dage) the player doesn't have enough energy after the boss's energy drain to cast effective shields and heal loop. If you were make the other two bosses have both types of weapons or remove reroute then blood mage would never be able to bring these bosses down even with energy parasite.
But even if this wasn't an issue here is the thing. For the legendary bosses the only build that works for them are heal loop, stall, tank (Insert name here) builds to endure their attacks as they burn their energy, and then chip away their health bar to earn the win. There is no way around this and nerfing these builds for the sake of strength builds (which are worthless against them) would indirectly buff legendary bosses, and change them from nearly impossible unless you have the right build/class/luck to impossible and here is why. The very same stats player use for defense and resist would be forced into energy for healing and shields meaning less stats to defend yourself with. This means the bosses will hit you harder and do more damge to you. This would lead to needing to heal more often and you only get one source of looping healing every 4 turns. you already have enough to worry about without this against them but with this unless you get extremely and I mean extremely lucky (getting constant blocks and deflections) you won't be able to beat them in this state.