How to get S rank in the last quest of chapter 3 (Skull Reaper boss)

[Vantroy]

AI always focus on attack the portal

[Morghen]

The only way I was able to do it was picking Sinon and breaking parts (they give you points), had to break the damn tail, the torso and try to not touch the adds or portal at all.

Fast forward some quest later, you'll have the same problem again and I haven't found a solution except stripping your characters of their weapon so they do 0 damage and you get more adds to spawn

[Phil~0~Sophie]

for me worked that i didn´t broke the portals instantly and "farmed" the monsters