It will take absolutely forever, but you are able to kill these with Argentavis. I spent the better part of 2 hours using a freshly tamed level 100 Argent to take one out. Make sure you're flying above the Titano and attacking it's back. It can't hit you very well from that angle. Land somewhere out of reach when you run out of stamina (killing things and eating their corpses for the health and stamina regen boost helps), and then continue. It takes forever, but it's definitely possible.

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