Nothing drastically different then the other guys said.

What I will say is if you can do the missions to take out the crusading nobles. Normally they land with a decent force, and join the faction. However quite often this your moment to strike. In my games they always grabbed a whole army from a city and left, grab two armies, set up a ambush, and hurl those javlines. If you win the rewards are pretty nice, and a good way to strike back.

Also while it is hard, if you can get your army near a russian army the Teutonic knights will attack you both unlike other factions. This is usually a win for you. Don't help Russia to much though, in one game I helped them beat back Teutonic order only to find them coming at me later on.