Geomancer + any other TM control can lock the boss down - worst case use Armiger.

Ninja + Achak can do wave lockdown.

Brakus to take down boss shield should do it.

Scyl, Apothecary, Alice, Ninja and Stag Knight.

Scyl aoe Stun and Alice AOE puts their skills on CD during round 1 & 2. Stag Knight does the dec def and dec atk (but they'll only ever be able to A1). Apothecary gets you extra TM and turns with inc speed. Ninja A3 for Rounds 1 & 2 primarily. With that setup, Rounds 1 & 2 shouldn't be any issue since you've got Negative Affinity in Ninja, Alice and Scyl but with all 3 combined you should be fine.

For FK Apothecary A1, Scyl A1 (when shield is down) Ninja A2, Alice A2 is TM control also and stag is just sorta there.

Now if you get Coldheart, she can replace Ninja or Stag knight.

Geomancer is a option instead of Stag Knight, especially if you can get him in a stun set. Rounds 1 & 2 will take longer but once boss's shield goes down it'll stay down

You are right.