I love the flamethrower - it is a weapon which requires a lot of skill, and is difficult to use effectively.
I find that creating a firewall in front of me in corridors pretty much always deters ANYTHING.
Not only that, but anything other than a tyrant and mara (Unless its just walking) which steps into this wall 65% of the time, ends up never leaving it.

's being the most likely to fall victim to an early grave.
Sure, this may seem obvious, but when being chased by three tyrants in a corridor on ATCS, its quite easy to forget.
When using a flamethrower, NEVER RUN.
I know that tyrant looks menacing, but running from it is the worst thing you can do (Unless you are 10 yards from your base).
Always try to just slowly move backward, and create a firewall in the direction of the tyrant.
Without charging, he will suffer about 180 damage worth of burns before getting into slashing range.
With slashing, he should get about 100.
By the time he gets this close, he should have about 270 health.
By slashing range, either...
1.Do not show any possibility of a chance you will die - continue spamming flames into the stomach while moving backward until either you or him fall, you make it to base, or it runs away.
2. (Only commit this if you are sure you can outmanoeuvre the tyrant)
Do a 180*, get behind him, and start spamming flames right up his backside until he falls.
Never run - the tyrant WILL track you down. I have learned this the hard way. Fight to the death.
As for other things, like goons, try to get infront of them as they are running away, or behind them, and use your flamethrower like a painsaw - they should drop very quickly if you catch them by suprise, and manage to get them before their reactions force them to pounce away.
With maras, dont bother.
They cant do much damage to you (If you have a BSUIT), and while it is easily possible to kill a mara with a flamer, just dont bother - it takes too long. If you DO still wish to do so, only attack retreating maras - estimate their path of retreat, and try to stay ahead of them while pumping flames into the air.
As for other aliens, well, you shouldnt have much trouble killing them.