Aegis Assault is great, and Hilstara can be quite useful in boss fights like Lustlord with a lot of single-target, high-damage attacks, though honestly my strategy for that fight is 3 offensive characters + Carina, and when he starts doing his OHKO lust attacks, Carina just revives that person each turn while the other 2 continue to nuke him into the ground. The way that healing abilities get insane speed bonuses helps mitigate Carina being one of the only slow characters in the game, and ensures that even after he starts using his self-buff to buff his agility, he acts after the whole party.
For most of the combat in this game, especially non-boss encounters, Hilstara's not that useful because the enemies do not make those kinds of attacks. When the most dangerous enemies spam fire breath or aoe 'pulse energy' type attacks, there's nothing Hilstara can do to keep that off the party. Similarly, even tanked up to the absolute max, there's fights where drawing damage to her doesn't meaningfully protect the party (for example, it's almost impossible for Hilstara to survive a round of combat against Skittering Nightmare with Shield Wall up, until you get multiple debuffs on everything at least: The best way to prevent your healers from being overwhelmed is to kill the lesser Dwaregast before they get to take their turns).
I play primarily defensive characters in most games, and I played defensively on my first playthrough. The reality of this game is that the random combat is best approached via maximum offense and some disabling. Boss fights are often best approached with some kind of defensive tool (stat debuffing and/or Carina's shield).
The later random combat in this game is balanced around your mandatory party member having a 2k group heal that revives everyone.
I've focused most of my commentary around regular encounters because IMO that's where people encounter difficulty and have problems with it. People expect boss fights to be challenging: they don't always expect random encounters to have a significant risk of wiping the party entirely, but in some areas of this game that is a real possibility.
The game isn't so hard that you can't use a wide range of tactics and be successful: I played primarily defensively for my entire first playthrough, using Carina almost all the time, using Vhala and Aka's debuffs extensively, etc. I just finished my 3rd playthrough, and I will tell you that my experience is that overwhelming, high-agility offense is the best way to reliably clear random encounters.
As I mentioned earlier, it's also a big advantage that you're often still bringing in more defensive or flexible party members for boss fights: You can have characters like Riala, Varia and Altina use their most powerful attacks over and over again and not worry about resource depletion because you won't be using them for the boss encounters anyway.
The purpose of my posts was not to suggest that it's impossible to do anything; it's possible. But if people are struggling with the combat difficulty in this game, my advice to them is 'kill things faster, and focus on debuffing or cc'ing things you can't kill fast'. And always use Robin if you can, because she's easily the best non-Simon character in the game from about the Ardford summit onwards (I have the summit provide basically Robin's entire gearset: Crusader staff/robe/charm + unperson circlet, eventually swapping the staff for an assault staff). Equipment-wise, outside of specific situations (like Robin's Magekiller fight, where using Otherworldly Robes is obviously insanely OP), characters should always maximize offensive statistics over defense in equipment choices (IE the Crusader Robe is the best in the game, and all the other casters should use lingeries, etc). A party setup of Simon + Riala + Robin + Varia/Altina will reliably kill 1-3 members of any random enemy party before they take their first turn if properly equipped. (Simon opens with Precise + Robin/Riala jointly target an elemental weakness + Blade Dance).
Depending on the boss fight, I pretty much always swap in Carina (for the Varia/Altina slot), often swap in Aka or Vhala (in the Riala spot, depending on whether the boss CAN be debuffed or whether you need a sex attack), only removing Robin for the very, very few fights where elemental attacks are worthless (Robin's Magic should always be so high that it doesn't matter if things are moderately resistant: She'll do more damage than most party members).
IMO, Qum is the most useful healer because of Pretty Lance, but most of the time you want a healer that isn't Simon (who has infinite resources, so he's your primary healer), that character will be healing every turn so Carina's better anyway. But bringing Qum for things that are susceptible to Lust is pretty good (though I'd usually rather have Riala's Sexual Lance).
In the later game, I find Hilstara and Yarra to be pretty poor and Uyae to be a frustratingly limited character - with strong attacking weapons, Takedown and Shockwave are quite powerful, but her best weapon options are pretty poor, and given there are segments where you are forced to rely on her as your primary healer, I have a tendency to set her up with double magic gloves, which makes her a relatively unimpressive offensive character, though Takedown can still be pretty effective). Yarra would really benefit from another 25 SP skill besides Haze of Lust - something like Pretty Lance/Sexual Lance that deals single-target damage and does more damage than Succubus Kiss/Sexual Torment. Incubus King's Emissary is wildly powerful but 50SP is too steep a cost for most random battle scenarios, as you end up actively farming SP with Masturbate for the next fight, which is slow and clunky. As I mentioned in another post, though, Incubus King's Emissary joins Bolt of Lust as an easy-mode button for certain scenarios where things that would surprise you are susceptible to Lust Death, including some optional bosses. There's nothing quite like killing a dangerous optional boss on the first turn with Lust Death, lol.
The offensive strategy is definitely extremely dependent on knowing enemy weaknesses: If you don't know what element to exploit to kill something in one turn, you really can't use that strategy. But it's by far the most effective and efficient way to tear through difficult random monster encounters like you find in some segments of the Tower. This is only exacerbated by how few AoE skills in this game are really significantly powerful: Even Altina's Quake is often pretty underwhelming. Riala's got the best AOE skills in her elemental lust attacks, bringing sizable damage and CC capabilities.
Party selection considerations and relative party member strength are relatively insignificant early in the game because you rarely have options: Your character choices are usually dictated to you by the game, and it doesn't matter if you want to use a given character because you don't really have choices. For that reason, it's definitely important to learn the most effective ways of using every character.
Another consideration for the max-offense approach is that it's pretty mage focused and consequently gets to avoid the very bad Evade/Miss RNG elements that you see with Hilstara's axes and, for reasons past my comprehension, Aka and Vhala even with their weapons having significant 'accuracy bonuses'. I don't understand how Vhala with a +100% accuracy Lean Unbow can miss so fucking often, but having her whiff on an Arrow of Abstinence you're depending on to kill something is sure frustrating. Varia still misses more than you'd like, but when you're primarily using a move that gets 5 swings (Blade Dance) the individual misses are less damaging - though I've definitely had Varia miss 2 or even 3 times in a Blade Dance and therefore not got the result I wanted, lol)
All of these 'preferences' are really out the door for the hardest boss fights where the ideal strategies are usually pretty specialized and specific.