When would it be good to use any of the tomebreakers? Aside from blue tomebreaker on M!Robin to counter Linde, I don't really ever l see any of them used.
When/on whom are they good?
Nino, Julia and Spring Camilla are all good candidates for green tomebreaker to deal with each other. Personally I have Julia with GTomebreaker and it was honestly one of the best decisions I've made. Pretty much never need to worry about other green mages. I don't think BTomebreaker is really used because reinhardt has brave tome and unless you are green or high res, then you are mostly dead so Btomebreaker has no effect anyway. Linde especially with Blarblade can oneshot a lot of the cast so again, unless you can tank a hit, then it's pretty useless. I think RTomebreaker isn't really used is because (at least for me) red tome users aren't very common. I rarely ever see Tharja or Sanaki
For melee weapon breakers, from what I've seen:
Sharena/Ephraim/Lucina/Nowi for sword breaker (Nowi should have Triangle Adept as well to lessen the damage from the common falchion users)
Reinhardt for lance breaker as I think it let's him secure KO's against certain Ephraim/Effie builds and natures
For axe breaker, I'm not entirely sure though I think it lets brave axe Cherche 1RKO Hector on the attack.
Bow breaker I think works best on slow mages (i.e Sanaki, Lillina, Julia) who can survive a hit from Takumi but fail to 1 shot him in return due to natures and skills. It'll also let them 1RKO Klein as long as they initiate attack.
Never seen dagger breaker probably because a properly built Kagero can one shot a huge amount of the common infantry units.
This is all my opinion though because at the end of the day it depends on your unit comp and what units you struggle against.
BTW gorl, did you complete Navarre lunatic in the end?
Anyway, I'm super excited for Xander and once he and Camus (really hope he's the next GHB after Lloyd) are released, I can finally finish my horse emblem team!
Also, who's everyone siding for in the upcoming gauntlet? I'm siding with Julia and I'll most likely make my rep Xander if I can train him up in time
Edit: Adding a bit more about GTomebreaker. I would say Julia has the most use out of it as she has high Res/atk (pretty sure she 2shots all Nino variants, though I'm not sure on buffed Spring Camilla) and is slow (without it Nino would double her and buffed Spring Camilla probably can as well) Assuming neutral Res and hp Julia, those 2 would need 72 atk to 1 shot, which isn't impossible but unlikely due to AI not being great at buffing.
Sharena/Ephraim would make best use of sword breaker as it prevents the faster sword lords from doubling them while koing them in return