If he's playing the HRE, archer stakes aren't really an option. However, the year means you've got plenty of good gunpowder, and halberds to hold the line, backed by Imperial troops.

Since all you are asking about is grand strategy though, I would totally rely on defeating them through a defensive strategy. Defend your cities and castles, and when they take a town, you can counterattack them. The biggest problem I have with the Mongols is usually related to their speed - I can only kill them when they are engaging, and they are able to get away when they rout. Trapping them in a city, or with a fort, makes this impossible and you can kill a lot of troops. So you need a mobile attack force just a little behind the expected line of advance.

Also, a massive assassin spam can be very handy as well, but good assassins take time to train. As the HRE, I usually shoot for an assassins guild in Frankfurt right from the start of the game.