If you want to fight and kill this guy in the wild, use the breath attacks on the lightning wyvern. However, it is highly advised to raise a high level lightning wyvern, fully imprint it, and level it into health, melee and possibly stamina. Stay above the giga and use the breath attack and regen stamina until the giga dies. Used this method to kill 3 gigas on Fjordur.

More Giganotosaurus Encountering Tips