It's almost impossible to kill a level 20 wild giga with a perfect 150 tamed giga, you need to breed 2 high level giga's and have a strong saddle to even stand a chance if you're trying to kill a wild giga. Remember that every wild giga has 80,000 health points.
If you encounter a wild one of these on the mountain you are trying to mine in, once you have a Quetzal with a saddle, these guys are actually pretty easy to negotiate with. All you have to do is get it to notice you, and then kite it downwards until it either slips the rest of the way down or finds a prey item. You will have gotten rid of the beast without even provoking it, and both sides will have parted peacefully.