which performing shigure and should i merge?

+Atk -Hp is the better bet for your shigure, as it helps him better then normal ones. As for merging, if you don't want to give Geyser dance to anyone then you might as well merge him.

My mindset is quite different from others I guess. I would prefer a neutral IV since with that spd and atk, even with boon he can't really fight, not to mention to be that frail as well. -Hp would bring both his bulk down quite a notch to lower 50s and he will be very very fragile. I personally use dancers for their dance/sing, and just to take a hit if absolutely necessary (distract AI).

My vote - it depends on what you plan to use him for.

As an attacking unit? Go for neutral, despite what people may say. As much as +Att is GREAT... unless you're going for the Death/Darting Blow build... it wouldn't be the best. LoD3 would be better for him, seeing that he's a glass canon in that form... and with -HP... he's at a higher risk. HOWEVER... if you slap Desperation 3 on him with that A-Slot? THEN I highly reccomend going for +Att/-HP.

Pseuso-healers and SP grinding builds don't really care for an IV, in all honesty.

In the end... the question is - what do you plan to use him for??
Decide based on that.