The Hakunos have standard AoE Buster farming specs—with a 50% charge CE and Koyan/Koyan/Oberon, they can tri-loop anything. Note, though, that even if Skill Reload lets you refresh skill 3 on turn 2, you’ll probably need to save it for turn 3 for loop reasons. This is the other reason the Hakunos don’t need Skill Reload leveled.
Assuming a level 80 NP5 Hakuno and a level 100 MLB Cranking, the Hakunos will do around 240k to Earth or Sky enemies on wave 3, a bit more to Man, and a bit less to anything else. This is enough for them to handle just about any node at the 90+-and-lower level, though they’ll probably never have sufficient damage for 90++. Unfortunately, unlike Ciel, they can’t really use their card rigging to help their own farming, as they have just enough NP charge to loop, so they can’t shuffle their cards without losing out on an NP, which will almost never be worth it, even setting aside the fact that their card shuffle tech isn’t totally consistent.
Still, though, being able to clear 90+ nodes is perfectly respectable for a free generalist.
As a CQ unit, the Hakunos can work in a similar team with a similar burst-oriented strategy. Their ability to control cards, though, gives you some added flexibility. You do have to be careful not to shuffle so many times that you break your loop, but you can generally afford to spend some of your batteries earlier in the fight in order to shuffle your cards and set up brave chains for Hakuno, since with double Koyan you’ll get 20% charge per Buster and (once you’ve stacked skill 2) 40%+ charge per Arts card.
The Hakunos are also a fun choice for Buster stall teams. Himiko/Hakuno/Castoria works in basically the same way it does for any Buster unit, with the key distinction that you’re much less reliant on card draws than you normally are in a team like that. By using Sparks Root, you can ensure you get whoever’s cards you most need, whether that’s extra Arts cards for more NP gain, Himiko Busters to leverage her own crit damage, or Hakuno cards to make use of their second skill. It introduces a level of interesting strategic decision-making to a team that’s usually pretty straightforward. Hakuno’s stuns help with sustain, too!
As a support, the Hakunos are in a similar position to Ciel and Summer BB—they don’t bring any party support other than card control, and it’s usually not worth sacrificing buffs for card draws. That said, if you don’t mind the fail chance, Hakuno is probably better at this than either Ciel or BB, since they split the difference between the two. Like BB, Hakuno can control cards for multiple turns, and like Ciel, Hakuno can (mostly) ensure a Servant of your choice draws their cards. They’re also more flexible in terms of positioning, since they don’t need to be plugged-out for the Servant whose cards you want (unlike Ciel) and their card control doesn’t fail if a Servant leaves the field (unlike BB).
So, when would you use this? There are two main cases: farming with cards, and getting extra cards in CQs.
Card farming is basically the same idea as farming with Ciel. Typically we don’t factor cards into farming thresholds because we can’t rely on them, but there may be some cases where being able to use Hakuno to guarantee card access might be more valuable than bringing a third support. Take, for example, a 3/1/3 90++ node. Normally, you’d approach this by looping all three waves, and you’d either bring a secondary ST unit (like Hephaestion) to help out for waves 2 and 3, or you’d just accept that you need someone capable of reliably doing ST levels of damage by turn 2. With Hakuno and a Buster setup, you can use your Koyan buffs and have your DPS NP on turn 1, plug a Koyan out for a Hakuno on turn 2, force cards to kill the wave of 1 with Buster crits, and then force cards again for an NPBB chain on turn 3. This lets you do things like run Black Grail (or a drop CE) on your DPS, since you only have to NP twice, and it can let you do more damage on turn 2 and maybe turn 3 than you’d be able to achieve in a typical Oberon setup.
This is only really worth it for Buster and maybe Quick, as Arts Servants just don’t have the card damage for this to be viable. It also comes with the big asterisk that you can get unlucky and fail to get the cards you need, especially if you’re counting on getting cards on both turn 2 and turn 3. Five shuffles should be sufficient, especially if you only need cards on turn 2, and any Buster DPS with party charge can give you even more consistency, but it’s still not a guarantee.
I’m also obligated to mention that you can apply the same premise to farming with Aoko’s AoE Buster. Unlike the Ciel/Aoko/Koyan/BB setup I mentioned in Ciel’s review, a Hakuno/Aoko setup lets you bring two proper damage supports, in exchange for being subject to Hakuno’s fail chance. My quick probability math says a Hakuno without a starting charge CE should give you about a 65% chance of getting Aoko’s Buster on the first two turns (assuming you’ll use her NP on turn 3), or a 52% chance of getting Aoko’s Buster on all three turns. A 30-50% fail chance is really not acceptable for farming, but if you’re farming with Aoko you probably aren’t worried about being optimal anyway, and event CEs with starting charge make the math a little better, so do with that what you will.
For CQs, support Hakuno’s inconsistency is a little less of a concern, since you can restart if you get super unlucky. Because the Hakunos reshuffle the whole card pool, you can use them to get a Servant’s cards more times than would normally be possible. This is nice for just about anyone—being able to NPAA or NPQQ or NPBB every turn is a substantial increase to damage and loopability—but it’s especially good for Servants who have huge buffs or high damage tied to their cards. Super Orion, for example, has his ceiling meaningfully raised by getting two or three Busters every turn instead of being limited to three per 3-turn cycle. Buster Servants will generally be the biggest beneficiaries of this, but some Quick or Arts Servants may like having Hakuno’s help as well. The big question is whether this is worth losing a proper support. Usually the answer will be no, but in some situations it may be worth it.
As a farmer, Hakuno needs a starting charge CE like Aerial Drive or Cranking. For CQs, you can also consider Black Grail for increased damage. For a supportive Hakuno, starting charge CEs are your best pick, since they give Hakuno more card rerolls to work with.
There’s nothing the Hakunos are obviously missing that CCs can provide. Crit damage CCs are always decent, especially on Buster units. Alternatively, you can give them buff removal CCs or the like to give them a bit of extra supportive utility.