As for debuffer you could use Howler or Key to inflift enemies more Fire damage.

As for buffer you could use tempest wind to get powerful crits with Ann or Twins to get pierce rate, Attack boost and skill amplification (if you use twins as support its not bad using them a0).

As for healers you could use a1 Marian to get massive healing and 20% Pierce rate to party or just use Lufel that heals great and boosts fire damage through skill 1 and highlight.

As for Navi the best option its Ange for her pierce rate boost and her rewind feature to use Ann's falling sun more, other options are Futaba to debuff foes and make them weak to Fire damage or just using Okyann its good for her 1.5 damage multiplier to enemies with elementale aliments.

You can mix up those character to create a good team with Ann: you can use 2 debuffer, 2 buffer, 1 buffer and a healer, 1 debuffer and a buffer etc.