For Antonella, make sure your friend unit has Earth/Light/Dark/Non-elemental weapons or damage is -50% or worse. Also, make sure you attack/kill in this order:
Ivy - B
Root - C
Leaves - D
Flower - A
Refia should open with Embolden(ATK/DEF) and Pecil with Focus (SPR buff). Rain should Full Break (Undermine) the target (BCDA), and then you chain/finish that target. Repeat Rain on next target then chain/finish. As for 6*, I would say Chiz, since she gets Phantom Shadow at 100, which is a GREAT 5-hit chainer. If you get your strong TGCid, have him chain and let Firion finish with Fin Briar. Noctis could be a good friend with Fire Flask, silence/confuse/blind/sleep could help a ton. Seriously, use the wiki page and watch videos of other people doing the fight with characters weaker than yours.
http://exvius.gamepedia.com/Attack_of_Antenolla
https://www.youtube.com/watch?v=DRvadynjDD8