General

@hiyoko: No problem, it was nice I could help you out. Gratz on defeating the Tengu boss!

@hualicious: Yup that was the key point about the 'S', I thought you must keep spamming the 'S' button instead of pressing the space bar once you were inside the purple aura bc 'S'= space bar :')

Finally defeated tengu 2 days ago, thanks so much for the advice!

@hiyoko: 'S' stands for Spacebar at Tengu boss! The NPC tells you about it (the chat that pops up at bottom right near your Reward Point window). Either the other players thought you knew what the 'S' meant or they got it wrong.

At my first post I didn't specified (now I did after editing and adding more detail) it either but I thought from reading it you might have figured it out too.
But yea it's more clear if people explained it: press the spacebar when you see an 'S' icon.

Good luck!

@hualicious: Thanks a lot for your advice, will keep it in mind when I reattempt Tengu!

@klu180: I'm confused over this, I saw some other people saying that you have to hit 'S' instead of the space bar to get the arrows?

@klu180: About your last line
Yes it is avoidable, and it's not the matter of fail or not. Even if you succesfully press the correct button, he will still do it. The main thing is: 'Don't let you get caught'. If you get caught the sequences of arrows won't show up anymore even after you free yourself from the boss, so that means an opportunity has lost.

Metinks that the aura appears after he does his flying thingy.

Also, your main problem is that you're hitting S. The S stands for "Space" and you're supposed to hit the space bar, not the S key. Weird, but Nexon is Nexon.

I think the OHKO is avoidable, but who cares. Just don't fail and he won't do it

-Is there a pattern to the purple aura appearing? Does it happen after every time he flies/uses whirlwind attack/screech that throws you back?

When the Tengu boss flies away and then it will make aerial dive attacks on you that you have to dodge, after he is done with that he will come from the background and flies right into your screen. After he has done that he will show up at the mid with a purple aura, so get ready and jump into the purple aura and press Spacebar as soon you see the boss appear, an 's' icon will appear which means you gotta press the spacebar button. If you stand there you just get half the damage taken but you can still press 'spacebar'. That's the only pattern for the purple aura to appear.

All the other times it appears it's random so you gotta pay attention to it. If you react a few seconds too late, you will miss out the opportunity to strike.

The boss doesn't necessarily make the purple aura appear after a screech (summons little crows) but the crows are getting in your way when you try to move forward (When I tried to double jump into the purple aura I hit a crow which slowed me down and made me miss). So like I said before all the other times it is random.

- When the purple aura appears, I try to get inside the aura and press 'S' but most of the time the arrow key to attack him doesn't seem to show or there's a popup saying Fail, am I not pressing 'S' fast enough/enough times? Must I get right to the centre of the purple aura to do it successfully?

When you press the spacebar in time, often an 'arrow' icon appears quickly. This arrow icon looks different and is different from the other arrows that comes up in sequences. So if you have succesfully pressed the spacebar and that arrow shows up and you failed it, it's okay ignore it, but the sequences of arrows that shows up after, press those correctly and you will deal damage. The arrow that you failed after pressing the spacebar, is an arrow that you have to press correctly to free yourself if the boss has captured you in its claws. So the fail will appear if you aren't even caught but if you were caught and you pressed it wrong you will die. If you press the correct arrow you will get released but you gotta run because it will stomp on the ground (he would have done the same if you were in its claws), which will 1HKO you.

-Right after I successfully get 1 'S' attack thrown in Tengu does the OHKO move of slamming the ground which makes my character die. Is this avoidable? The main thing is: 'Don't let you get caught'. If you get caught the sequences of arrows won't show up anymore even after you free yourself from the boss, so that means you have lost an opportunity to strike. To avoid getting caught, if you are outside the purple aura, jump into it either to the left or right side. If you are within the purple aura, make a jump to the sides of the aura the boss will miss you and don't forget to press Spacebar of course. By the way you don't really have to be inside the aura but if you jump and passed the purple aura you need to receive the 'S' icon, you can succesfully press the spacebar outside the aura. I have flash jumped through the aura and landed outside the aura and still managed to press the spacebar successfully.

Press the correct arrow and jump away.
Edit: Sometimes the boss will act weird, like in the middle of Aerial attacks it suddenly stops doing it and shows up and do other things (like whirlwind attack or others) and after he finished that he will continue where it stopped with the Aerial attacks.