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Often times those S tier units are either 5 star exclusive, or require premium skills. While tier lists are certainly helpful, I wouldn't worry about trying to compose a team of S/A tier units only. For this, they want either Speed to double their opponent, or guaranteed followup. If a Galeforce unit starts with the Special counter at 4, then they will need to attack twice per engagement. Double opponent (do not defeat them in the first hit) Activate Heavy or Flashing Blade.
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These are my current 5* heroes: Genny (+SPD/-ATK), Ninian (+ATK/-SPD) + Water Blessing, Ayra (+ATK/+DEF/-SPD), Sword Lyn (+SPD. I need to know which heroes I should give ally support to so I can have better team synergy. I've been playing FEH for a while now, but I'm still having trouble with higher level battles. Dragon-effective weapons, such as Itsuki's Mirage Falchion, are also very useful since most of the high scoring armored units are also. In upper tier scoring for Arena (19-20, or 19.5), you can't go wrong with armor-effective weapons, such as an Armorsmasher, or utilizing units with armor effective prfs like Caeda.
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