DO NOT use a tamed Giga to kill a wild Giga. Tamed Gigas gain a massive nerf to health and attack and lack the ability to use the gnashed debuff like the wild Gigas can. If high lvl, imprinted and with a high quality saddle, you might be able to take on a low lvl Wild however.

Treat Wild Gigas like bosses, use a well leveled and saddled army of tames specced into melee and HP. Wild Gigas have 80k HP and 500 base melee, for reference a base Rex has a mere 1.1k hp and 65 base melee. Alternatively a mother load of auto turrets with a ton of ammo protected by metal gateways can eventually kill a Giga.