I didn't assume an yth's arm would be three feet long - I assumed its armspan, the entire distance from the claw-tips of a right hand to the claw-tips of a left hand while the measured pair of arms is spread as wide as it can go, to be three feet... So yeah, I was picturing a [i]much[/i] more short-armed - and by extension, only half as mobile - yth.
- An adult yth would have an armspan of ~208 cm, whilst only having the body length of ~152 cm? Interesting proportions for a non-flying creature of this size, and ones that would probably make moving around and balancing somewhat harder. Their arms are positioned attached quite close to one another on the body, right? ...And now I feel somewhat tempted to try sketching an yth, if you don't mind?

Neverteless, moving on... I don't see how Aemoten could have done much better with all the obstacles around, beginning from not hitting any of the guards to the wall on his right and the other people behind him to Olan's halberd, held by the yth, being somewhere in the way. (How did the rightmost guard end up partially under the middle guard when the middle guard fell first? Wouldn't it be the other way around?) 
...So we have an yth which is still stuck by one of its right arms, with maybe three quarters of an inch deep and four inches long cut on somewhere on its hind body from Aemoten's first swing at it while it was in air, with shallow cuts on the underside of its upper right arm and top of its middle right arm from Olan's halberd with its middle left arm missing, its two other left arms with flesh sliced off on the outer side near the shoulders, and a shallower long gash somewhere further down the side of its body?
Nothing immediately fatal, yes... Although I've got a feeling that the yth would be losing very large amounts of blood with all those injuries, the missing arm especially - with a relatively large creature such as the yth, a proportionally long limb like that usually has quite significant blood-pressure behind it, amongst other things. (Not to mention blood-vessels of smaller diameter are easier for the body to seal even with the same blood pressure. Earthen creatures who can easily survive losing limbs are small mostly for the aforementioned reasons.)

Is it safe to assume the yth would retreat farther from Aemoten and try to remain facing him rather than try to attack immediately? (Meaning it would move over the guard to its right that it is still stuck to and retreat onto the floor, with the people who were in front of it (remaining halberdiers, Olan) being farther to the right in front of it, Jaelnec being somewhere to the left next to it, the stairs being to the left in front of it, Aemoten being in front of it, some tables and chairs being behind it?
In that case, Aemoten would draw his sword to a low defensive position (so he can block if the creature decides to throw itself at him) and close in again, delivering a much less restricted, essentially a low-horizontal half-circle swing (base of blade higher than tip).