Against world bosses, you just accept that you'll lose most battles and do as much damage as you can before you die. A general who does 100k damage to a world boss and dies of reflect damage in the process has done her job. When you see someone get a score of 30 million or whatever in world bosses, he didn't do that in one battle. He's burning a bunch of fast battle tickets to do dozens of battles in the half hour allotted.

It is possible to keep generals up a little longer with large amounts of lifesteal, such as Miyamoto Musashi's skill. But that necessarily comes at the expense of dealing more damage.

If the problem is corsair enemies reflecting a lot of damage in the campaigns so that you can't get an A rank, that takes lots of healing. The healing lab skill also helps here, as it doesn't change the course of the battle itself, but does lower the threshold of what it takes to get an A.