You do realize that minato doesn't always rely on his kunai. He can make marks on anything. A rock would be his kunai. A leaf of tree could be his kunai. Not only this, those marks move. If the sand of garaa were to be marked, that mark would move to garaa. Plus, he has kunais in many far away places, not just with him. By the time, either one of the kages notice minato, he/she will already be stabbed by a knive.
One scenario: tsunade gets marked, minato suddenly disappears not on the battefield but minato teleports to his house which would be far away from the battlefield. Every single one of the kages would be concentrated on the kunai left by minato. no one will know that tsunade got marked and she won't know about it too. (remember, the mark moves, it's extremely impossible to sense if you have a mark). That mark allows minato to stop time and space. This means that minato can teleport and stabs on tsunades Vital organ. Tsunade will only notice minato when she has already been stabbed. (obito didn't noticed minato until a knife was already inside of him and a contract seal was already done on him). None of the five kages would be able to do anything because they can't attack since minato is so close to tsunade and if tsunade tries to make a move or anyone tries to make a strong attack, minato can just teleport and the only one who would get hurt would be tsunade. He has a reaction far greater than any shinobi.
It's true that with combinations from the five kages, they might be able to stop minato's kunai but the combo's he can use with his marks, kunai and other abilities is limitless