So I got lucky on the legendary banner and got a second New year Azura, should I merge her with my first one or keep her for hone fliers fodder?
I also got a second Shiro the first one is +att the second one is + def. Both are -sp. Which one should I use?
If you're going for a melee enemy phase build with shiro like steady breath/ stance and quick riposte, then opt for +def. If you're going for a distant counter build, go for +atk to maximise your one shot ability. Since both are -spd quick riposte is essential in either the B or S Slot. As for Azura, unless you're going for a high merge with more azuras and want to use her in arena, I would keep her unmerged and see what else you pull.
If you like fliers, hone fliers is gold or even more precious. Way more helpful than +1. If you're planning to go +10, maybe that's a different story, but then why not let her take up a single space in the barracks until then? Can help immensely with setting up the first attacker so that the rest can rush in without worrying about extending out of goad range.
Also, the previous responder gave me the idea that you might at some point want to put double of her on a team. Most people would just use multiple alts of her, but you might find useful in some unusual situation.
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