This is just a guide describing how to fight the Gnoll Chieftain. If you want more details on the Perilous ruins or anything else other than how, then look in other guides.
First of all, Understand that all enemies, bosses and normal mobs alike, go through three phases. A targeting phase, and an attacking phase. The attacking phase is always followed by the vulnerability phase, which is longer or shorter depending on the attack. Do not attack during any phase other than the vulnerability phase, or you will leave yourself open.
The Gnoll Chieftain goes through those phases too, he has three attacks. Knowing them would be nice so that you could recognize how you need to move to avoid them.
[SIZE="4"]The Kick[/SIZE]
Avoiding the first of his attacks is rather easy. He just kicks. He only does this if you're close, so if you keep just out of reach of his leg, he won't bother with this attack. Be careful though, this attack comes fast and is often hard to dodge or block. It doesn't do much damage, but it has the habit of keeping you pinned down in front of him while he continuously kicks you. Be wary of that. Also, the vulnerable phase for this attack is very very short. Just wait for a better opening if he uses it.
[SIZE="4"]Hammer: The Vertical Strike[/SIZE]
A vertical downward strike. It lands pretty far away from him, so don't run away or he will hit you. After this attack, he taunts for a moment, that's his vulnerable phase. This taunting period is the best part of the battle to drink a potion, use a chain hook, or use a feather. If you use a feather or potion just as he finishes the attack, you will be done before he can pick the hammer back up to try and chase you. It's also the best time to attack because he'll be taunting for a long time. Finish your attacks when he stops straddling his hammer.
[SIZE="4"]Hammer: The Windmill[/SIZE]
The spin attack is his signature newbie killer. Like the vertical strike, the Chieftain moves forward quite a bit doing this attack. Running directly away will not save you. This attack is more annoying because it leaves you with very little space near the chief to be attacking, and he picks his hammer up more quickly with this attack, the vulnerable phase is shorter. Don't use potions or try to revive after this attack because he might kill you while you work (You can usually use it during the spin and be fine, but it's troublesome, so just don't do it.) Instead of restoring, use this brief period to get off an attack.
If you use your normal attacks to move closer, by the time you're in range you'll have a stronger smash already prepared. Fiona can use Ivy Sweep, only try Amaranth Kick if you started your combo early, otherwise it's too slow and he might try to kick you. Lann can use Twin Edge 450, or Thousand Needles. But if you're going for Thousand Needles, only use a few needles. If you try to use too many, you'll wind up vulnerable.
You can identify which of the attacks he'll use based on his posture. Both feet will be on the ground for the vertical strike, and he raises his left leg when he spins, but it's sort of difficult to stare at his left foot while you fight him, so there is another, much more easily recognizable way to determine which attack he will use. And, as a bonus, using this method will both automatically prepare you to dodge his attack and position you for your attack.
The second method is to run in constant circles around the Chief while staying out of range of his kick. If you can do this, you can solo this mission on Hard with Season of Macha even at low levels. The reason I want you to do this is because, when he uses the vertical strike, he will follow you. In which case, you just keep running (Not sprinting) and he will miss. Then you're already close and you have plenty of time to pull off whatever strong attack you feel like. When he uses the spin, he won't follow you. In this case, you run directly toward him.
I know it may feel awkward moving towards the enemy while he is using his most devastating attack, but just trust me. If you don't trust me, then remember, The Chieftain moves forward while he spins. He moves forward in the direction that he is facing, while you are moving the the opposite direction. In a lot of cases, you won't even need to dodge. (But I like to dodge once I get to his side.)
Oh yes, that reminds me. You're not moving to collide with him. You're moving to come up to his side. Since you were running in a circle, you're already offset to whichever side you were running in, so just move so that you're a little bit behind the chief and he'll totally miss you.
The problem with doing this to dodge the spin attack, though, is that it leaves you really far from the chief. Lann might be able to hit him with Twin Edge 450, but Fiona's attacks might be too slow, so try to get a head start if you play Fiona.
There's a plus to being this far though. Remember how I told you that you normally couldn't use potions or feathers because he's faster to recover from the spin? Well now you can, because the distance between you two takes time to cover.
I generally like to use the time after a spin attack to use restoring items, and the time after a vertical swing to attack, since you're so much closer and there's so much more time. (By "so much more time" I really mean about a second or so.)
If you keep on like this, the chieftain will never hit you.
[More info. Will add pictures later when I get them.]