Mongolia is pretty annoying, especially when your tactic is allying with as many CSs as possible. His UA is focused on taking over CSs so if he goes after one that you are allied with, your choices are to try to block his path, bribe him into making peace or declaring war on him. I used to try and gift units to my allied CSs but they are even worse than Civs at combat and will embark their units as soon as you gift them.
There has only been one occasion where gifting units has worked for me - it was late game and Mongolia were still around. I had two ally buffer CSs between me and him and he declared war on both. I had great unit production so I churned out loads of Xcom squads and para-dropped them into CS territory and gifted them (lots of them). It was so much fun seeing the mighty Mongolia beaten back by little City States!!