A- Support Rank: Lose points equal to a bonus from what would've been an A rank support.
I think at that point, we just ask "Can we just bring one of these two, instead of both?"
Also, Serene has a Base Damage of 14 with her Fire Tome, so if she can double him, she'll be able to do it, possibly, if the Fighter has 5 RES or lower. Fighters don't have high RES, so I'm willing to believe they don't have enough, but I might be wrong. Also, not so sure about the Speed of Fighters, which is another factor in allowing Serene to finish off the Fighter.