Note: This strategy is for 3.2, I don't know if it works with the newer version.

After many failures I have finally found a way to successfully play as the Orcs of Gundabad. It's no secret they're one of the hardest nations to play as. It's impossible to avoid a two front war at least to some extent. The key is to focus on the Dwarves to the West. Here's what you do:

First, take all the units from Carn Dum, including Gordoff, and send them to Gundabad. Do the same with the orc fellers in Gram and everyone in Litash besides Lunul. Take everyone in Gundabad including the general, recruit a couple of of snagas from Gundabad and head East. The target is Azinbular-Dum. Stick North as much as you can until you are directly above the Dwarven castle. This will take like ten turns to get there so be patient. Once you are above the Eastern Mountains, head South until you hit Azinbular-Dum. At this point I'm the garrison should be miniscule so you should be able to take it without much trouble.

While this happening, you need to secure your borders to the West so you can hold off the armies of Eriador and the Dwarves to the West. Take Ladbug and everyone left in Gram and take Coldfells, recruiting all the mercs you along the way. Then circle up to Athilim while building watchtowers around the borders of your territory. These are essential so you can keep an eye on Eriador on the Dwarves, otherwise they can easily surprise you. Take Athilin and use it as a buffer zone between your capital so you can see enemies coming from far away. If a force too great approaches, abandon Athilin if you have to so you can muster up troops back in Carn Dum. The High Elves shouldn't bother you this early on just leave them be for now.

Once you have taken Azinbular-Dum, now comes the hard part. Holding it. The Dwarves are gonna be pissed and throw everything they have at you. Build a Shrine to Melkor and then a temple right after it to maximize your culture spread so you can begin retraining and recruiting troops as soon as possible. Build watchtower on the direction of Erebor and keep your spy hanging around Erebor so you can see what's happening. Fending off the Dwarves will be tough, but I was able to do it and I'm not exactly the greatest Total War player especially in battles. Do not let them besige you. Wait until they come within reach and then take the fight to them. Dwarven units in sieges are OP. It does not favor you. Just make good use of your archers and Trolls. After a few attempts the Dwarves will ease off for a bit allowing you to replenish your forces. Most of the buildings should be built already, but another great thing about this castle is it'd in the mountains, meaning you can use it to recruit Trolls. You will need them to eventually take Erebor, which will be a as I'm sure you know. Make sure you have a full army before you attack with at least two troll units. The good news is because the Dwarves are using all their resources to take back Azinbular-Dum they won't be able to expand West and threaten Gundabad, so that's that problem solved. The Misty Mountains Orcs will even move East and likely take Wormcove.

Once you take Erebor the Dwarves in the East will be neutralized and you can focus on expanding into Eriador. It's worked twice for me, once on Medium and once on Hard.