HP in battles has never been an issue for me, regardless of using Defense/HP oriented units, except for dealing with mosquitoes; the only solutions to those buggers are Toad Lily, or some first-aid with Edelweiss.

At the level caps and max affection stats, units that focus on Defense over HP are often able to reduce damage by up to 50% against regular mobs and around 25% against the Bosses of 12-5, when compared to other units with a difference in Defense of 300 points or more. This usually leads to Atk/Def-oriented units, such as Cepha, holding up in battle just as well as Paulia.

BUT THEN, there's also units that just never die, like Di Yu and Mountain Lily, not only because they have a million points of Defense and HP, but also because their abilities are buffing their already absurd Defenses to the max. This also happens to be a prime example of how I prefer to arrange my squadrons. Except for some, most units usually break out even in terms of fighting power, so it's usually their abilities that set them apart, which is why I'm always concerned with the team synergy, instead of stats.

From there, I have to be on the lookout for units that can maximize the use of abilities..........or those that don't. Glomphrena is my prime example, as she has a Defense ability and while on paper, has a good base amount of Defense to utilize it, doesn't get enough out of the Affection Bonus to actually utilize it efficiently (on top of having fragile HP).

Damage bonus to the Solar Drive is also very worth mentioning, because of how the damage is calculated. Unlike Attack or Defense buffs, which adds all of the % Buffs into a giant pool if you place multiple stacks of these abilities on a squad, damage buffs to the Solar Drive actually MULTIPLY ON TOP of each other, using 100% as the base starting point. So for instance, if we could stack 5 Lavenders onto a single squad, the power bonus wouldn't be 100 + 100 = 200%, but 100 x 1.2^5 = 248%.