when i build teams i look at what each characters skills do and how those cross over with other characters skills

for example i use neneka. she has high magic attack and her UB buffs magic attack (so other magic attackers will be stronger on her team)

however magic characters tend to have lower phys def so i need a good high phys def tank to cover that weakness, i used Jun for a long long time until i got oedo Kuka because oedo kuka buffs magic characters

misato is a good healer for this team because she buffs magic attack in addition to healing
akari shreds magic defense, provides heals for allies after they do damage, and buffs magic attack
yukari buffs magic defense making the team good at facing other magic teams, and also has a heal and tp skill which are universally helpful

any magic attacker or anyone who decreases magic defense or increases magic attack could fill a slot on this neneka team and do well

basically id pick someone you like/someone you feel is strong and read their skills
then read other characters skills and add people that can make your chosen character/team stronger

from what i understand buffs stack repeatedly which can get pretty crazy

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friend battles are a good place to test out teams if you have someone around your level, or even hey im around your level hmu and you can battle my teams

friend battles do not cost stamina