I'm not surprised you were having issues if you were rocking 3 fire guns.
You need to mix it up
fire vs flesh, electric vs shields, acid vs robots/armour
TBH, I had the exact same issue as there seems to be a limited choice of elemental weapons in the first dozen or so levels.
Of the decent guns I've found with elemental damage, the majority had fire damage, some have had electric damage and 1 had explosive damage. I've not found any with corrosive damage, which is a bit galling after suffering though 3 areas of mainly robot enemies
