There's two "Yoinks" isn't there`?
Anyways, to get the 15 units thingy.
Most city states have 1-2 combat units and a worker.
Lets say a CS has 1 combat and 1 worker.
Move a considerable amount of units, 13 in this case, into their borders, even embark some into the water tiles if there's not enough room.
Gift all of your units.
Marry the city state.
Profit!
What will be a chock, atleast it was for me is that all your promotions you had on the units prior to gifting them away are gone.