Ah yeah. Fire Emblem, a great game indeed.
If you take a really close look to the critic attack of Lyn you will actually got it. The first slash is going from down-right to up-left.
That will be the first slashing image starting from the left of the sheet combined with the second. So, we can say that the second slashing image is a continuation of the first one, and that makes a single large blow.
That's why the end of the line in the first image is fat headed, so the second image can be placed parcially on top of it. Same thing happens with the third and fourth images, (from up-rigth to down-left), they both are a single great blow.
After the enemy get's hitted, she then returns to his place, and that's the last slashing image.
Reasons for making a first blow in two images could be a couple, I think.
One of them is to make the eye believe that the blow really last a while so, it gives an impression that Lyn traveled a long trail around the enemy in pretty high speed.
She is really fast, so she cannot be seen! D:
The other reason is that the width and eight of every image that represents the blows were not enough, so they had to make it in two pieces, probably, because some systems work better in power of 2 images, say 64x64, if you take a look to the second and fourth images, that are the continuations, they have the same width and height, and the fourth one Its like is not finished, but it doesn't matter, because, where is placed on the screen, it make's it look like it continues.
And about the spikes, I'm not an artist, but I think it resembles where the end of the sword is pointing, notice that, spikes are when she draws the sword from the cover, and at the end of the blow, when she hit's, giving some impression of colition or friction.
And the middle clean part is where the sword takes more speed.