To be honest, I was completely lost at first, because no matter what I do, nothing seems to happen. But I finally got it now.
Sex Battles only happen to monster girl bosses (Like Succubus), and your partner (via EroEro)
There are 2 phases in a Sex battle, seduction phase and insertion phase. (Sorry for the names, I'm bat at it)
Seduction phase
Before attacking, you need seduce the girl, and the only way to do it is the talk command (2nd choice in battle)
There are various scenarios given, all that are completely random. Some scenarios include guessing (?) what items they like, giving gold (humans), giving levels (monster girls) etc.
Successfully seducing a girl raises affection, which is shown like this
Unsuccessfully seducing a girl is shown like this
To deal damage, you need to reach a certain point of affection. You can check if you've reached the cap by looking at your weapons. If the dmg is high, and doesn't increase anymore, it's already at maximum. If it has no dmg at all or it only deals low dmg, you need to seduce them more. Also note that you can also use weapon skills in this phase.
Sword - Moderate Dmg, 5% crit chance. Highest Accuracy (85%)
Spear - Lower dmg than sword, 10% crit chance. High Accuracy (80%)
Axe - Highest Dmg, 20% crit chance, Lowest Accuracy (60%)
Bow - Lowest dmg, 25% crit chance, Moderate Accuracy (75%)
Some girls can also deal HP dmg in Sex Battles by the way, so proceed with caution. Some can also drain your EP immediately depending on the choice you make (like giving you a fellatio).
Once you've depleted atleast 75% of their, you can now go to the insertion phase. Depending on the choice you made, some girls have different poses for each insertion phase
Insertion Phase
In this phase, neither you or the girl will "die". Instead, your goal is to cum all over her as much as possible. Depleting around 65% of the girl's EP will give you the choice to cum. The first choice will always make you cum inside her. You will lose 1 point of LP everytime you successfully cum, and both you and the girl's EP will be maxed. After the first ejaculation, you will be given a new set of choices on where to cum, namely
The final choice is for bonus purposes. Fully depleting the girl's EP before cumming will give bonus EXP at the end of the Sex Battle. The more ejaculations, the more EXP is given as well.
*Some girls can critically damage you, losing more EP.
*You can do sex battles with your partner in the brothel. It costs a bit more, but more exp is given.
*You can replay monster girl Sex Battles in the Succubus Territory