You need to set 4 money bringing minions on last row. Just set 3 and when you'll have money for another one- set him on that row. After that you will earn 20 gold/sec and thats enough to set 2 slimes for each fire attack. Just be carefull to set slimes everytime when one of your slimes dies. That's how I beat the level :> 

Bro... Fire attacks are so broken. How the hell do I even beat that level? They just come out so fast and kill my minions too quickly

the game is so hard, you need to adjust the damage that the enemys can heal so its possible to beat the game

oh god level 3 is too hard bruh