From the angle it looked like the plan was to let the force of the onrushing fish impale itself, rather than time a slash to coincide with its onslaught. As for choice of tools, perhaps it's the only sharp one available to her. As for grip, I don't know what is standard for mermaids or whatever she is. She may have just found that sword 5 minutes ago from a shipwreck, and had no time to practice.

Remember, the theme of the comic is that she's not some super-special "lady of the lake", and instead is more just a girl with a weapon in her hand, or at least she did for a brief moment! So her not being prepared seems to go hand in hand (so to speak) with that.

None of those details are likely to make it into King Arthur's (fishy) tales that he will tell all his friends about "how I became king, once upon a time...", especially as the tail (ah... see what I did there!?:-) grows with each retelling.