Some quick strategy tips:
1. Keep enemies out of the zone of their color and most will be unable to hurt you.
2. Most skills do full damage even if you target enemies outside of the heroine's preferred zone.
3. Get F*cked more. A heroine will recieve a temporary x2 damage buff if you destroy the enemy she's having sex with. Keeping enemies alive in their zones with low health is a great way to farm the buff and turn the tides in your favor.
4. Keep an eye on your HP bars to stay on top. If possible try to not have all 3 Heroines have low SR at the same time. Don't be low on Stamina if you've got lots of enemies in their zones.
5. Planning your route. If you've started a round with skills you aren't very good at using. Try your best to make it to the shop so you can purchase more skills. If you feel pretty confident, challenge a mini boss to get some stat upgrades. If not, you should probably take a different path. If you think you know who the final boss will be, purchase items you can use to keep yourself on top.
Let me know if these tips help you win some more of your runs.