Canned suggestion with the caveat that they may not necessarily be the “power” plays, is to roll for favorites. You don’t need anything to complete this game, up to and including the most difficult of challenges.
Summer BB’s defined by her gimmicky card-lock S3, which is generally more of a “win more” button for already developed roster, but she’s a passable generalist DPS as well courtesy of hitting everything for neutral. Jarcher is the only other AOE Arts Archer aside from Helena (Archer), and she trades high NP damage for unusual bulkiness and at the highest end in terms of farming being able to run “6 slot” (or, CE, specifically) comps, although those get expensive by deign. Worst issue, again, is her relatively soft NP damage despite her carrying all sorts of buffs. Either could improve your roster by simple virtue of being SSRs.
Aforementioned “power” plays are the following:
- Tamamo (Caster): THE Arts support, generally built towards slower playing teams.
- Scathach-Skadi: THE Quick support, generally built towards faster evisceration of the enemy but can last beyond that too. However, she also has massively broad uses outside of her Quick-centric comps too.
- Merlin: THE Buster support, and a great generalist support as well as his kit as everything (healing, invincibility, etc.). Has a lot of uses outside of Buster or crit comps too.
- Zhuge-Liang/Lord El-Melloi II/Waver Velvet: The all-purpose support, has something everyone appreciates in universal buffs.
- Nero Bride: Arts support with generalist capabilities due to her similarly universal buffs, including a notable 45% NP Gen Up that later comes with a 30% static charge.
- Sima Yi: After her buff in two years joins Waver and Skadi in the “50% charge club,” more built towards supporting Berserkers but has uses past that. Currently unreleased.
Bedivere is an amazing nuke to have at NP3 for you. Siegfried is a capable Dragon-slayer and general anti-Lancer point.
Euryale and Orion both excel at nuking males, though Orion doesn’t falter outside of Males at the cost of more durability, slower play, and a more packed kit.
The Cu’s are solid undying Lancers, and LAlter is an effective if bland charge-based AOE Lancer. Her 50% charge is the defining aspect of her kit, and it makes her a solid farmer that can potentially NP twice, though she otherwise has very little overall utility. Still, having her alone narrows what you “need” for convenient anti-Archer farming.
EMIYA is an effective if clunky outside of his ideal comps (due to trash NP generation) anti-Rider unit.
I suggest looking at the “welfare” section of my thread here to see which units we will be getting for free alongside Ishtar (Rider) that can fill gaps for you. For example, next summer has Jeanne d’Arc Alter (Berserker), basically Lu Bu 2.0, i.e. a really solid all-purpose high-damaging stat stick that can serve you very well. Other elements of that thread are worth looking at as well, such as the SR/SSR chances of success spreadsheet I linked.