In general: B!Micaiah works well with fliers and bond users, since (if you use her default kit, of course) she’s also a bond user
Aversa is a great choice since her tome debuff the foes stats and she can buff Micaiah using hone flier on her C slot (technically her default C, res wave, is also good). Plus, Aversa can be more tanky than Micaiah, this means that she can kill strong DC foes without being killed
Overall, you should create a flier team that can buff Micaiah and help her during the battle. L!Azura is also a really good choice but she’s rarer compared to other units
In Arena: When you play Arena things are different. You should use the bonus unit of that season and “cheerleader” units. A cheerleader can be a dancer (HS!Micaiah, L!Azura, Reyson or even free people like Olivia and Ninian) or a buffer like Marth, Tharja and M!Corrin. These units have to help your bonus units, because when you play Arena you must kill the foes with your bonus units to earn more points. While B!Micaiah is a great unit, she doesn’t really help the other units, so instead you should use someone like Aversa because, like I said earlier, her tome debuff the foes and this helps a lot your bonus unit
B!Micaiah is much more useful in AR since she’s a flier with a great colour. Green is useful against foes like Ophelia and Reinhardt
If you need something else, just ask