Killing a wild giga with a tamed giga is suicidal, unless you own a max level pair with almost 300 armor value for the saddle and both are fully imprinted. I only recommend this method against low level gigas, around levels 1 to 5. Anything higher and you need to swarm them with fully imprinted high level gigas with better saddle armor. Many of them will still die, but they'll get the job done.