This takes place during the story after you have went to see Alisha after coming out of the Underground Ruins in Ladylake. You have to fight Oroboros in order to continue with the story. I was playing on Hard Battle Difficulty and what I found was that the Boss only followed me around that battlefield and kept his focus on me. Dodging his attacks while letting both of the Seraph characters focus on damage and healing made this fight a breeze. Make sure to watch out for his rising coil attack which hits in an area around him and can take out other characters besides you.
Make sure you have fused some of your equipment to at least +2 or +3 with skills on defense and health, and you should have no problem since Lailah can heal the group on her own or if you're in a bind armaitize with Mikleo to help with the healing by using elixir vitae and damage him from a distance with the arrows but take into account that Lailah won't be healing you as much because she uses the fire element.
You should also take a look into the battle actions if you have unlocked the one that lets you armatize after K.O. it can help in a bind but just remember that it takes an additional character off the field that can add as a healer or attacker.
If you're having a problem with this boss you can grind the lesser enemies in the area to gain levels and new artes but it shouldn't be all that difficult.
Here's a video of the fight if you need a little more help with it. Credit goes to Vylsith for making it not me.