I have NO idea why on earth I haven't done a guide to Germany yet, but here goes.
Objective: To pepper his army with crossbow fire and hit and run and whittle down his military and economy with long range fire, finally going for the throat with uhlans.
Crucial cards:
Age 1:
2 settler wagons
economic theory
Age 2:
9 crossbows + 2 uhlans
3 settler wagons + 2 uhlans
700 gold + 2 uhlans
700 wood + 2 uhlans
Uhlan hitpoints, + 2 uhlans
TEAM hand cavalry attack (team games or 1v1 when out of better cards)
2 caravels + 2 uhlans (water maps)
Age 3:
Cavalry combat + 3 uhlans
Fort + 3 uhlans
13 jaegers for 1000 gold
8 skirmishers + 3 uhlans
9 uhlans
TEAM 2 settler wagons (for team games of course)
Age 4:
Lipazzaner cavalry + 4 uhlans
Factory + 4 uhlans
Factory + 4 uhlans
Those are the crucial cards. You can add in others, too, especially if you're modifying the strategy.
Age 1:
Standard age 1 stuff. All vils to food, explorer scouts + gathers treasures. Make 1 or 2 houses, depending on starting wood, send 2 settler wagons as first card. Age up with 400 wood. While aging, switch your 5 settler wagons to wood. Send 1 or 2 vils up to make a forward base.
Age 2:
Throw down a barracks as fast as you can and send 9 crossbowmen. Once you get your barracks, start spamming crossbows and, whenever you get a free moment, shoot at his army or vils and run away. If you're not fighting his army, you should be looking for vils to snipe. This keeps the enemy on edge. If he is making light infantry, engage him with your crossbows and charge in your uhlans from the fog of war and panic him. Until you get more than 5 or so uhlans, this will not be very effective, as uhlans are fragile and die before they can do significant damage. If he goes cav, add in some pikemen. If he only has a few, though, just hit and run. Your third card should be 3 settler wagons + 2 uhlans. A good rule of thumb is to put all settler wagons on wood (They shoot and walk more slowly, so they're bad hunters), and all normal vils on food. It balances resources nicely. Your fourth card really depends. 700 gold if you need to make uhlans or age up, 700 food if you're going to continue to spam all crossbows. If you begin making uhlans, you may want to send uhlan HP, too. Put a lot of vils on gold, as uhlans are very gold-heavy. When you get the resources, hit the age 3 button. If on land, do 6 skirms. If on water, do the barbary corsairs and the privateer.
Age 3:
Send cavalry combat as your next card if you've begun producing uhlans by this point, or send 13 jaegers if he has heavy infantry and you have extra gold. Those guys are awesome. They can snipe vils in 5 hits, kill most heavy infantry in 3, and have 40% ranged resistance. Anyway, obviously get veteran uhlans and crossbows, and begin considering going for the throat. You could spam cannons, heavy infantry mercenaries, or continue annoying him like some sort of German fly and age to industrial. Just wing it at this point.
Age 4:
Send factories if you need artillery or lipazzaner cavalry if you have a lot of uhlans who need more power. Once you get all the cards and the industrial upgrade, your uhlans will be more powerful than imperial status. That means over 380 hp and 75 attack! If the game isn't over at this point, just keep making villagers and uhlans, throw in some cannons, etc.
Tips:
Obviously, raiding. Use your explorer to look for villagers (especially hunters), dead animals, falling trees, etc. If there's a big group of vils out in the middle of nowhere, send your uhlans to scalp them all. Otherwise, use your crossbows to pick off any villagers you find. With your high base damage and decent range, you shouldn't have a very tough time. Just watch out for his army.
Dangers: Watch out for large amounts of cavalry! Uhlans can serve as anti-cavalry due to their high base damage and crossbows are decent against cavalry, but you still need to watch out.
MICRO MICRO MICRO. The Germans can NOT stand up to other civs at all in sheer numbers of units. You need to watch your guys carefully and use your soldiers better than the other guy uses his. That's the only way you can win.
When you finally decide to go for the throat, consider heavy infantry mercenaries (landschneckt, swiss pikemen, highlanders, ronin, etc.), cannons, or sheer masses of uhlans. Uhlans have 20 base siege damage, which isn't great, but later on you can begin to spam them and they become decent at tearing down buildings.
If your uhlans come across a few heavy infantry while raiding, don't be afraid to fight! If you vastly outnumber him, you can beat his guys down with your high base attack and continue raiding with minimal casualties.
I'll elaborate on how to effectively destroy massed light infantry, which most players do. (Russians, French, British, Aztecs, etc.) Send your crossbowmen into them and start trading blows. With your high base damage, you should hold your own. Meanwhile, send your uhlans around back in the fog of war and charge into the back of his infantry. If he has any heavy infantry, target them with all your crossbow fire. This way, his guys are pinned. Do they run into your crossbows away from the uhlans or into the uhlans away from the crossbows? Either way they die.
Anyway, I hope this helped! It should be a pretty good strategy, as it got me to where I am using the commonly accepted weakest civ in the game. Remember, have fun with this! If you only care about winning, go play Iroquois or China or something.

Good luck playing the most terrible but fun civ in the game!