Their fight is so terrible. Screw Canoneer. If the fight was just blade bearer, it would've been a perfect fight. But then you have the fat **** spamming fireballs nonstop, completely ruining any semblance of proper boss design. It's easy to nullify one's element but it's too aggravating to solo this fight because you get knocked around constantly by the canoneer even if you make it so it barely damages you. Imagine if Smough in dark souls just never stopped spamming projectiles. Nobody would care about dark souls if they made decisions like that, which this game seems to like making all the time.

My way of dealing with them:

Transform your veil to ice, use zweihander with fire transformation, use any anti ice buffs you have and any fire attack buffs you have.

Focus on Boobstein first as you'll want her out of the equation as quickly as possible. Your ice buffs will negate most, if not all, of her main ice damage so you can focus on using the zweihander's high defense to block the incoming physical damage while taking chunks out of her health between attacks. Guard reversal helps but imo doesn't last long enough for it to be useful. Try to keep knock-off Smough in you peripheral so you can dodge any incoming attacks, and keep an eye on the ground around you. If it starts to glow, get the hell out of dodge. With your fire buffs, she should go down before long. Once she's delt with, turn your attention the the big guy and try to stay behind him. A good ice weapon here is useful but not all that necessary - as long as you switch from your fire weapon you'll be fine. If he surrounds himself with fire, back away as quick as you can to avoid some heavy damage from the resulting explosion. Keep at it and he'll fall too.

lol so easy. killed both on 2nd attempt as a dark mage
yakumo aggros on blade bearer while you spam guard of honor ice spell to nuke down cannoneer
after, spam ember reversal on blade bearer which absolutely destroys her. super easy fight.