Megid...instant kill to a Grinna Bete C on Desert Goliath S (luckiest shot ever o_o).

In terms of actual damage...I'd say either Diga (highest actual damage modifier), Foie (the burn adds a ton of damage on the bigger enemies), or Dambarta (easily the fastest way of clearing groups of fire enemies such as Badira/Koltova/Ollaka).