I happen to be an expert on this topic. I'm going by the old jap version here so take my skill naming with a pinch of salt.
-the game is very easy and scales quite badly unless you max it to Nidhogg Berserk 4 at which point it gets hard and transforms into something quite unlike the capture-and-consolidate it's been up until that point. It's some of the best fun I had with any Venus Blood game but expect to sink some time into it, needing both a maxed-out army and a LOT (like minimum 10 million) gold, although you could just hex-edit the gold.
-For most characters the best setup is going to be helm-split and 2 extra attacks (Godslayer/Godspeed). Reducing enemy defense doesn't seem that great until you start raising the difficulty.
-The basics of a squad are either to have a tank in slot 1 with Defense Formation (this is what causes interception of melee attacks) and a source of Range Invalid so they can't shoot past the tank, OR a low-def decoy in slot 1 being covered by a tank with Foward Defense OR tank in slot 1 and decoy with Evade 100 in slot 5 OR tank in slot 1, decoy in slot 5 and an off-tank with Evade 100 and Forward Defense in slot 6. There's only 3 chracters that can do Evade 100. This is important because Dimension Slash is awful (defence-reduction like Helm-split, stacks with it, and ranged but Ranged Invalid doesn't stop it) and Evade is the best solution to it.
- Targets are chosen simply by multiplying the Defense and current HP of your six squaddies and aiming at the lowest. You can create a reliable decoy by giving them a skill that ruins their Defense, not re-hiring them so they don't get a boost from battle experience, and by giving them Exclusive Defense or failing that a ranged attack.
-Those Evaders three are the Imp, Amazon Druid, and Tiruka. Give her Forward Defense as a bonus title, all the other options are garbage.
-Tanks should have Exclusive Defense (half damage taken, no attacking). No attacking is good because they don't hit hard anyway and you don't want to provoke counters. They should also have Slayer Defense (no multipliers from racial type) and if they can fit it in, Night Battle Adaption. Mostly they can't because they also have to help provide:
-Cross invalid is the ability to prevent multiple hits across your ranks or a penetration front to back. You generally have to do it seperately because there are few sources of actual Cross Invalid. This is a priority when building because it will wreck your shit. Asura has this skill and despite being supposedly a tank is actually a single target dps, so you can have at least one proper tank who doesn't have to worry about trying to do two jobs at once. I'd recommend the Elf Swordswoman, surprisingly enough.
-If you don't have penetration-denial, avoid putting a Blade-type behind your tank. Lancers do them 50% more damage and most Lancers have the skill.
-Activity stacking (bonus to stats of X type in same squad)is important. It's less so if the squad is focused on spell damage output because they're not really using Attack or Speed.
-Don't be afraid to put characters in roles they don't seem to be designed to do. If it works, it works.
-Every squad should have status protection, preferably the single one-size fits-all skill for efficiency.
-Magic defense is nice, but unlike later games you can't really get it that high, so just make sure you have one squad with the demon chick (Astarte?) who has lots of it, and make sure that's the squad you toss at the rare magic-heavy opponent.
-Don't stick nocturnals in with non-nocturnals. Their Defense stat will fluctuate and ruin your decoy setups.
-You can combine self-destruct with Recovery and low defense to make a suicide bomb squad that explodes twice. It's very effective but it will also cost a lot of gold.
-Despite not having Exclusive Defense, winter loli is still one of the best tanks, partly because her corrupted tactical skills include a massive defense booster and heal. She needs a ranged attack and probably an Evade decoy, though.
-If you do intend to win on the hardest settings, you're going to find most Lancers fall off hard due to not having access to S-rank skills.
-that gaggle of free officers you get at the start of the game are actually surprisingly good even if they don't seem like it straight away. The dulluhan kind of sucks apart from the fact that you lack tanks early on and the kobold is a solution to a problem you don't really have in Frontier (auxiliary tactics, morale formation), but the rest of them are very useful, so keep them around. The wolf in particular is about the only S-ranked lancer who doesn't suck. The kobold is at least useful as a generic healer.
-Stack treasure hunting, use that to get treasure hunting items, remake into a better treaure hunting squad, and you can have Rank 7 items before you leave the first chapter.
-You don't actually need Rank 8 or 9 items, but they may help to patch holes in setups when you've done 9 squads and are kind of binraking for the tenth. They look awesome until you get far enough in the game to access them and realize they're not actually as useful as they look in most cases.
-Woodland types are pretty useful since they tend to have very few other type-tags and the terrain they do badly in isn't that common. The only good tanks they have are the Elf Swordswoman (if you get cross attack invalidity from elsewhere) and the Barbarian (who isn't that great), though.
-The demon knight palette-swap upgrade has a very rare full-area attack invalid skill. You might want to put her in with Astarte. Just saying.