The best luck I’ve found with flamethrowers in PvE at least, is juggling enemies with the rifle and the flamethrower. If you time your cooldowns well, you can net, knockback, switch to flamethrower, napalm, knockback, switch to rifle, jump shot, net, knockback…
It’s tedious and only really works 1v1, but I’ve all but soloed a few random champion mobs doing it, and take very little damage.