rn when i play the campaign i only ever do all bonus brutal, and i play the mission its intended to, aka i try to use the unit unlocked there as much as possible(with few exceptions like zhurou, kayvillon etc). just gonna give some random thoughts on the missions.
Walchice(stinger), Juliburg(seawolf corsair), Fel Diacia(tb), Sirlanna(griz). Maps i used to think is hard but gradually as i get better i feel much more fun in place of hard, they are fun and good practice material, i just go back and play them randomly from time to time. all are missions that require you to expand, multitask, micro and destroy enemy in time.
Caderical(zenith), Ciphe(starlight), Chatka(merlin), Intrepid(pala), Karuwel(scorp), Rockbar(reef), Mannia(detriment). Maps that are macro heavy and slow, which i likes alot, all of them are fun and easy, something i can just go in and chill/do whatever i want.
Old Falsell(crab), Sapphire(deso). i really like these two because there are very little missions that i feel like have a good design for just porcing and defending. Although Old falsell is not designed as a porc mission but it is also very unfair simply with how much the ai has, it just feels way more fun to play it as a porc mission. There are few defending mission in the first place, and then some "defending" mission like Walchice or Lalata still provokes you to wipe out the enemy as usual.
Tempest(ulti), Pendust(shogun), Bluebell(skuttle). other kind of hard maps.
Ganong(Domi) is the single hardest mission in the entire campaign if you try to do all bonus, there are four diffrent, distinct bonuses that you have to do... not only you barely get anything are the start you also have to micro every single domi, not to mention thier terrible handling. not complaining, but im not good enough for this mission, this is the only mission that i've never finished all bonus brutal in one go.
Zhurou(drp), Kayvillon(siren) this two are pretty hard, but different to a "skill issue" thing like the above, these two are just out dated missions that just became obsurdly unfair over the balance changes. can be frustrating if you dont play it with the right mindset, but still can be fun.
Bavhakya(likho). absolutely terrible design. the odin killing trinity is fine... but why are you telling the player to spam likho on a narrow map where it is also one of the maps where the ai spams aa? and ofcourse the ai simply have way too many stuff, this is especially true as likho has a "efficentcy limit" where it takes forever to reload.
and more importantly is the bonus... why the hell would someone want to snipe the commander? it does not do ANYTHING for the opponent, and it is compltely RNG that if you can kill it or not. i remember retrying the mission 20+ times to just ONE match where the commander comes to frontline at a possible-to-kill timing, and then i fucked that up so it is 100% my most hated mission.