Stat Thresholds!

Heavy Blade-55+
Flashing Blade-40+
Wind/Water Sweep-40+(30+ if I use Phantom Spd seal)
LnD if there bulk is trash(B! Cordelia for example) Swift Sparrow if they have decent bulk( Soleil for example)
Ploys-35+
Panic Ploy- 50+
Infantry Pulse-45+

In my opinion, for default stat:
Heavy Blade: Really high atk, probably around 52 or above
Flashing Blade: Should definitely above 40, better if you reach 44-45 or above.
Watersweep: Maybe around 45 and above if you want to count in for the fast mages. If you're only looking for watersweep-ing dragons, perhaps 35-40 is fine.
Windsweep: Also around 45, considering fast unit+DC roaming around in the arena. If you're only looking on crippling armored DC units' HP, 35-40 is fine I think.
LnD: If you can reach like ~50 atk and ~40 spd with LnD and you're just using this unit for player phase, that's fine.
SS:For more bulkier units, SS is better, and it performs more or less the same as LnD. Just more hard to find tbh.
Ploy based on res: I like it really really high. at least 40 is convenient, but above 35 also works fine by me.
Panic Ploy: HP around 50 and higher. No buts.
Infantry Pulse: As long as you can make it higher than the rest of your main infantries, it's fine.
Brazen skills: I think it's around 50 or less if you want to use ardent sacrifice. Reciprocal also works fine though it requires a bit of basic math.
Well, probably there are many better opinions out there. But, an opinion is an opinion right? :v
Thanks for reading if you do.

Edit: I mistook SS as Steady Stance, not Swift Sparrow, sorry about that.

- Heavy blade: 55 is likely enough; 52-54 is not as reliable but probably 60-80%. 63 is close to 100% chance of activation.
- Flashing blade you probably want to double the opponent anyways, so the threshold is not how much to activate flashing blade but whether you can double them.
- Watersweep: this one is kind of nasty. Dragons are usually slow, but mages can be so fast. Depending on how many merges you expect to be up against, 45 might be fine, or 50 might only be like 85-90%?
- Windsweep: most armours are easy to sweep; Draug is probably the exception. But if it's some sword, even if it isn't Ayra or Karla or Mia, 46 seems like a reasonable bet to me. 48-50 feels pretty safe.
- L&D vs SS is not so much a question of how much you have to begin with as what the unit needs to do. Desperation mages probably want fury because they want more bulk. Firesweepers probably prefer L&D because they want to keep just enough bulk to survive a single hit if they need to, which means they want maximized speed on both phases. SS is surprisingly hard to find a niche for, because even if you're avoiding L&D to try to land ploys, fury might be better. The best use I can find for SS right now is with the recently released summer weapons, or if you have exactly 55-ish bulk to tank a counterattack. (Note the correspondence to the heavy blade number.) Arguably, fury is still better because 1 atk 1 spd for 3 bulk and stat boosts active on both phases feels like a good trade to me.
- Ploys beside panic: this was asked in a topic around a week ago. 30 is more than enough if you're targeting res-dumpers and glass cannons (including mages such as SM Eirika unless she gets fortify cavalry). 35 is a pretty solid number to target blade mages (based on my recent study of blue infantry mages); 40-41 will hit anti-mages such as Micaiah and fury Felicia/Kaze/Innes even with 3 (or maybe even 4-7) merges unless Micaiah uses fury or they have a res boon.
- Panic ploy: 50 will hit HP-dumping dragons; 52 will hit non-HP dumping dragons unless they have merges; 53 will hit mostly anything without merges. If you're just going after horses and fliers, 45 will probably do it.
- Infantry pulse: shouldn't you be planning your team before you decide who to fodder it to? But in general, there are two types of infantry pulsers: those who give it to everybody, and those who stack it on the low-HP units. The latter category only has HP in the mid 40s so that they can receive the infantry pulse from the highest HP units. The former category usually has HP in the upper 40s or even 50+.
- Brazens: 50HP is the max you can have and activate it using ardent sacrifice, but be careful because blessings or bonus unit boosts might put you over. Brazens often combine with 75% threshold skills (desperation, vantage, wrath) so 40HP would be the max - again, don't forget that some things will push you over. I was the one who asked the question about getting rid of HP using assists; the consensus seemed to be that this tactic is slow anyways because somebody on your team has tanked an attack which means the battle is under way, and you're throwing away a turn just to get below a HP threshold.

Anybody want to offer insight into brash assault?

heavy blade: pvp should be 55+ before merges but pve content like chain challenges and story mode only really needs 52ish
i miss the days when heavy blade worked perfectly with just 50 atk...
flashing blade: 40+ after refine and before merges and A slot
water/windsweep: 35 is fine if you plan on using phantom spd, otherwise, same as flashing blade
LnD: def/res bellow 27 SS: Def/Res above 27
Ploy: 31 is decent enough for low to mid level arena
panic: 50+
Inf. Pulse: most units taking advantage of inf pulse r very glass cannon and have low lvls of health anyway so i'd say 40 before merges/refines/skills/seals
brazen: if a defensive variant (def/res or atk/def) then i'd say more is better but if offense variant (atk/spd) then 40 is a good cut off