Good game idea, and the flamethrower is fun to use :D
As a lot of comment pointed out already, the bot is not very useful at the moment, mostly because you melt everything (I mean, YOU HAVE A FLAMETHROWER! ^^). I think you could address that by making the enemies slower but more durable, and getting pushed away from you when hurt. It will give player some form of crowd control.
The other issue is the AI durability. I tried to spare it, but it wouldn't last more that a few seconds. Give it some HP! And maybe a pickable that only the player can grab that will heal it (for full on cooperation), and make it spawn only sometimes? I think the player need some wiggle room for mistakes.
Regarding the rest: The music is okay and I like that it stops when the AI dies. The graphics are simplistic, but clear and readable, which is really important! And i think that, in the long run, you'll need more enemies and maybe more levels
But hey, I'm a simple man, you give me a flamethrower and things to burn, and I'm having a good time ^^