Bosses

Usually they appear at the end (or near the end at least) of each update. Some of them are alone and others have company. Usually they don't drop any kind of special loot or reward.

Riala
Withered Mountains' Fortress

Your little party has to beat two Elite Succubi and a powerful mage. She is too much strong to be defeated at that point in the game (except by cheating), so the best advice is you should not spend any of your consumables in this fight, because you are meant to lose this fight.

Cache Guardian
Incubi King's old cache

It will spawn after Yarra joins the party. For your level, it's tough but you have a full party and it is vulnerable to Lust attacks. Be aware that it can apply poison to whole party with Foul Blast.

Chimera
In the trail

It only uses its most dangerous attack (Foul Blast) when its health falls below a certain level (at least in my experience). Even then and depending on the level of your characters it could be a menace, so don't pull your hits.

Lead Slaver
Meeting hall

Unremarkable in all the aspects. Providing that your entire party is in good health, you shouldn't have any kind of troubles with him.

Portal Demon
At the academy

It convokes more enemies as the fight progresses, but it's better to concentrate the attacks on it, before it has time to use more than once or twice its most devastating attack (the onslaught of Energy, an AoE spell that could kill all your party, except Simon).

Slaver Ringleader
Slaver warehouse

He has a good punch, but is vulnerable to the Poison Blade (you should have this skill by now). Also, he doesn't seem to have any kind of AoE. Beware that his minions using frequently the Heal spell (although not the Cure one so they could poisoned with immunity).

Seed of Corruption
Sineford's Mine

Not particularly difficult or powerful. Apparently it can't be poisoned but its loneliness helps. Doesn't seem to have any kind of special attack.

Ogre
Devil's Pass

Its AoE skill (Whirling Club) is not powerful against a rested party, but in otherwise it could generate some troubles but nothing unmanageable. Don't forget to loot the corpse near for a Dwarven Helm. The experience that you get is pretty nice too.

Noble Scion
Ari-Yhilina Palace

The key to winning this fight is to eliminate the Scion first and then focus on a single wizard. They are tough customers, because they can heal themselves in one turn, so keep the pressure on them until their demise. Defeat the scion using lust attacks (he is susceptible to Yarra's Sexual Torment attack). He drops a Shining Sword.

Unhuman
Desecrationist faction's house

He has a very poor agility, so will probably attack before/after Hilstara. His Inhuman Aura magic attack affects the whole party, but I hadn't seen any other AoE spells. He shares with the other Unmen their weakness against Lightning so a Level 21 Robin is a formidable asset against him.

Varia's Second
Varia's Lair

This fight could be a cakewalk or something more challenging in function of the level and the composition of your party. If your Simon has already reached level 26, the possibility of using AoE skills like Simon's Commanding Presence, Yarra's Haze of Sex or Altina's elemental storms it will take care of all the minions in a couple of turns.

Stone Gargoyles
Old Fortress

A couple of tough gargoyles that attack when you try to access the basement. They have very high HP and are resistant to fire but they are weak against ice. They can be specially if your party is low in mana and/or health. They give more than 4000 XP so they are good to level up the party before the arrival of the Horde.

Aramite Elite
Eustrin

A tough Aramite Soldier, accompanied by four soldiers. As both Carina and Varia are relatively frail, it is recommended that you use Hilstara's Shieldwall and Carina's Shield of Purity. Among his attacks he can Blind or Silence a single target, so equip your party with the appropriate Pins. After that try to raise your defense and target first the normal soldiers. You get 3790 XP and 1052 Sx.

If you have a Reshaped Varia, then both she and Hilstara can use Encourage. Having both of them use Encourage on Varia on the first turn means that Varia can use Furious Strikes on Turn 2 to do a formidable amount of damage per hit, possibly one-shotting up to three guards if she has good enough gear and even taking down the main soldier in one hit if she scores a good crit. For defense just have Carina spam Shield of Purity twice and then top off anyone who was hit too hard.

Esthera
Empress' Private Training Grounds

You had to fight to her thrice in the last part of the Simon's visit to her country. Each time, the fight will be tougher. The first spar is pretty easy. My Simon was not a top of his health so I combined Precise Strike and Health Potions and I beta her without trouble. An alternative strategy is to use normal strikes and not special strikes with suppress lust when you got 10 SP for healing. Use precise strike when you got enough SP pool. You heal more than you can be damaged. You get 3490 XP for this fight and Simon's Skill Suppress Lust it's converted to Lust Renewal.

The second one is tougher. It's possible using the Lust Pastry that you should have got helping the Office Supervisor. If you don't have it, try to use a Large Health Potion. It's perfectly possible to win this fight without any healing items if you got a massage from the baths first- the health regeneration effect of invigoration will buy you the extra turn you need and you'll want to visit the baths at some point because you get an RP bonus with Nalili the first time you visit the baths after sleeping with her, so it's the perfect opportunity. For the offensive, don't bother with Unified Strike, just use Precise Strike. You get 5825 XP and increase Simon's lust Gain.

As it happened with Riala in the Prologue, you're not meant to win the third fight so no reward if you are able to. In any case, you learn a new Skill, King's Aura.

Optional Bosses
These fights are not obligatory to advance the plot, but they are a pretty good source of XP and Sx or other goodies.

Heart of Horror
Devil's Pass Optional Dungeon

This is a tough bastard TM. It has no special resistances or weaknesses and it cannot be silenced, blinded, stunned or lusted (it can be poisoned but the odds are against it).

Some of his techs are: a single target damaged+paralyzed, a whole party damage+blind+silence, and a whole party damage+blind. It gives more than 3000 XP and comparatively few money (little more than 600 Sx). Defeating it, doesn't end the dungeon (so you still have time to eliminate any other enemy still alive).

Demonic Essence
Ari-Yhilina's Lower Town (Warehouse's basement)

You only find this boss if you had found the mysterious Cult. It has two companions with less HP and Mana. The boss has about 10000 HP and it has a dangerous AoE that really damages your party, but fortunately is vulnerable to poison. So the best strategy is to poison the three of them as quick as you can and to try to keep the three members of you party alive as many turns as you can. You'll get about 2300 Xp from this one, plus a pitiful of Sx and a great Robe.

Wards & Conducts
Ari-Yhilina's The Order of Yhilini Thaumaturgy

Once you had passed the three trials at the Order, you have the option to test a new Conducts test. It's a very challenging fight and the only reward is 2640 XP (and the admiration of its creator, that could help you in the long run).

The tactic described above was shared by Seil at the BKG forums.

As your party can be overrun easily, you have to prioritize the elimination of the enemies above doing general damage. Of the six enemies, conducts A and B are the least dangerous, so you give them less priority. On the other hand, Conducts C and D are, on the other hand, very dangerous but they are vulnerable to physical attacks so Aka could lead with then one by turn with Forceful Lunge. Meanwhile, both Robin and Altina should use Lightning attacks against the Water Ward and/or trying to keep the party alive. Once you have dealt with these three enemies, the rest should be doable. You can use Ice spells to defeat the Lightning Ward.

Concrete Demon
Ari-Yhilina Miner's Junction

To unlock this option you need to speak with the elven mage near the loyalist bar in the Lower City, and he'll give you a spell that lets you amplify the effect of the demon summoning before collapsing the rift in order to make the mine unusable by the noble houses or something. If he won't give you the spell then go do other faction quests and leave dealing with the mine until last. At any rate you then need to go to the northernmost room in the mine and fight the humans guarding it, then choose "Use the amplifying spell" instead of closing the rift. Note that this will throw you into the boss fight immediately with no opportunity to heal up after fighting the guards, so be sure to do your healing before you end that fight.

The boss itself uses physical attacks exclusively and can one- or two-hit your mages with its biggest attacks, so it's incredibly important that you blind it with Aka as soon as you can or else you are going to be in for a wild ride. After that, have Aka use Disabling Assault twice to soften it up and weaken its hits and use Forceful Lunge after that until you need to reapply a debuff. The demon seems to resist all damage types equally, so just use your biggest guns and have your mages heal if things get too dangerous, but don't stop piling on the damage.

This will annoy House Adamant so you won't get paid for the job and it may have consequences down the line but it will also make you more popular with the commoners and may offer different profitable opportunities as well. If you're worried about the consequences, keep a separate save slot with this fight as the last thing you do on Aka's route and with Aka's route as the last thing you do before Simon's route so you don't have to backtrack as much if you want to change things.

Fallen Chosen
North-east of the succubi village near the Gathering complex

The following strategy has been proposed by Omega2501 in the LTS blog. It's a sounding strategy.

Equipment: You should have some kind of lust preserver for both succubi and if you got the cache as one of your investments give Yarra the the charm of perversion (the yellow trinket) which gives her the Masturbate+ skill. Orcent should have the best gear that you can get with your available Sx.

Have Orcent spam his Defensive Attack, rotate two Defensive and one Piercing Attack or just the debuff active on him and use Piercing otherwise. Qum should be healing or buffing the party with Arousing Aura. If she can do neither, order her to Attack. Yarra should be either attacking or casting Sexual Torment most of the battle. Don't forget to use her First Slut ability if she knows it as it gives her buffs which are quite useful.

That is pretty much it. The fight will take a while, as he is resistant to your damage, so you don't do HEAPS to him keep your SP (the red bar) up for Yarra with the already mentioned Masturbate skill and you'll get there eventually.

You get 2309 XP and 1000 Sx plus a Shining Sword for your efforts.