Light Grid is by far the most developed, with Light Herc Axe as well as a Phantom staff, two LB3 light weapons from last UE, Reshep's Fire staff LB3, two SR disaster hammers as well as the dark SSR from panel as well as LB0 Michael and Vishnu weapons. SR light himes are Diana, Attar, Sati, Artemis, Hermes, Uranus, Durga, Sukunahikona, and Belenus.

Fire Grid by contrast is far weaker, with 2 Resheph's staff LB3, Marchosias's arcane LB3 and LB0 Amon, Prometheus and Amaterasu weapons being the only fire weapons. The rest of the slots are decked out with whatever high stats I can pop into place (aka all light weapons). SR fire himes are Culyune and Motu (yes, I know, absolute shit, but elemental advantage... maybe?)

To answer your final question, I haven't tried out Icarus yet but last time against Reiki, the most common cause of me losing was an AOE wanpanning everyone across the board, leaving no one to tank for Vishnu.