A few notes on the 1.1 flame thrower:
It does 100 damage / second with 150 ammo. In raw damage it only loses to the lucifer cannon (I don't really know what the damage/second is but meh) and the painsaw both of which are arguably niche weapons. For me the flamer is mostly for crowd control of smaller aliens though your mileage may vary.
Either use it to farm a higher stage or together with a buddy to achieve some specific goal... You could create a wall of flames at a door to keep the smaller bugs at bay while he blasts critical structures inside the alien base with a pulse rifle.
Generally speaking try to create walls of flames instead of aiming at any specific enemy. If you are fighting a marauder guess where it is jumping / falling and create a wall there. For dretches and basilisks create defensive and offensive walls of flames. If you are moving be aware where you already have flames so you do not run into them. That means if you are running backwards and firing do not start running forwards

For bigger bugs lets be serious... It is not meant for them... Go get a shotgun or whatever you personally prefer for big bugs.
My 0.02€ cents.