Team comp advice ( and how to use it )

Wows, sick amount of Tanks you've got there :P
[Ahem] I would get Effie to 5star and use her to smite Hector around that would transform him into a mobile fortress [he gets to move 3 tiles this way] (obviously with exceptions to weapon triangle and Red Mages) as for a healers I actually prefer Serra over Elise (just for the aura Atk up) but since you already have a 5 star healer just use her instead, Elise is much more mobile(Not that you'd need it when using 2 tanks) and has more survivability when compared to Serra and she can indeed be used as a magic tanker/bait along with Takumi the setup should be rather stable.

You might also have to create a red swordie team with Hector and Takumi just for arena. I've been encountering more and more Hector setups in arena lately. [Has yet to confirm if this is due to my rank or because I've got a Hector, I'm rarely chatting on discord with my peers "Work work"]

Side note by using Hector, Takumi, Elise and Effie combo as defense setup you would very likely cause your opponents to weep jello. [Set Elise as leader for troll bait]
(Welp, unless it's me lol I'll probably bait Elise with my 14 Res Takumi, use Effie to charge up my specials for both my Takumis and have your Hector duke it out with mine.) But like I said before the AI in this game plays our characters in the worst possible manner I guess the safest defense the game has to offer would be Hector, Hector, Elise and Takumi or your defense setup which I mentioned earlier.