Tips for trying single player expert:
1 - When fighting the horde, always stand behind your bot teammates. They have better accuracy, but you can be smarter.
2 - Givin the choice for any of the 3 grenades, your choice should usually be boomer bile. Why? It keeps the horde busier than the others. You don't need to kill the infected. You just need to get by.
3 - Always make wide turns whenever possible. Jockeys, hunters and chargers rarely miss from close range.
4 - Conserve your ammunition and all other items to the best of your abilities. Things can go wrong VERY easily, so keep everything handy.
5 - At the end of the round, if the AI is bleeding out, finish them off so they start with 50 health in the next round. Do it SAFELY though. Make sure you don't kill them and get carried off by a jockey or something...
6 - Let the AI do most of the shooting. They cannot accidentally shoot each other, nor can they damage you from shooting you. It only takes one clean shotgun round to the head to incap a teammate.
7 - You should be using the shove action more than anything through the entire play. Keeping a melee weapon handy and using the shove/swing/shove/swing cycle can keep you at 100 health.
8 - Keep against walls whenever possible. When fighting the horde, try to keep your back to a corner. When moving across areas that are difficult to see, hug a wall to the best of your abilities. Even if it's out of the way. Better to do it the right way then the quick way.
9 - Learn the pathing of the level. Your speed is perhaps your greatest defense. The infected might run as fast or faster than you, but if you take sharper turns when running from infected, close doors behind you, etc. It will work wonders.
10 - Kite the tanks. Undoubtably the toughest part is dealing with the tank. It only takes one swing or rock from it to incap you, so keep moving. As soon as the tank is chasing you. Keep it in sight, but head all the way back to your starting safe room if you must. While it's in sight, your bot teammates are most likely shooting it.