The Battle Chest where you're outnumbered as the Plant team and your 3 companion AI teammates aren't all that great was a tough one.
I tried it a bunch of times as Acorn/Oak but found I was just too big of a target and I spent a couple of hours working at it. I rotated through a few characters until I finally managed to get through it solo.
There's a bit of a trick to it and a large amount of luck involved as well.
Some of the things you can control and other parts you can't but what I noticed worked well in my favor was relying on characters with good range.
The sticky part is that you'll likely encounter two or three Zombies at once so you want to keep your distance and preferably stay near at least one of your AI teammates- preferably with them between you and the Zombies so they draw more fire- if they get vanquished but you finish off the attackers you can revive them if they don't stupidly respawn without waiting very long.
The bridge tends to be a hot spot for encounters so I tended to stay to one side or the other, depending on where I spawned, with a preference for the side with the two story building and the tanker trucks. If you're alone and at one side of the bridge you can use cover to hit Zombies as they cross from the opposite side. A good vantage point can be the second floor of the building next to the cheese tanker trucks if you tend to get ambushed from behind while on the bridge too often.
If you can group with the AI and move together make sure you keep back a bit, again so the AI draws fire, and assist the AI in taking out any Zombies you meet and then reviving fallen teammmates when the area is clear whenever you can.
You'll want to do your best to vanquish at least two Zombies yourself before you get vanquished- the more the better. Distance between you and the enemy can be the deciding factor since the enemy AI doesn't fire flawlessly at long distances so if you're a better shot than them that'll give you the edge.
It'd definitely a tough battle solo. If you happen to be on PS4 I can help if we happen to be on at the same time- I'm realitysquared on PSN.