Switch between your seal and your main weapon.

My seal is usually in my offhand. Is there some sort of button to swap your active right- and left-handed weapons quickly?

For the horse, you can swap weapons just like you do on foot. However, "Offhand" seal in these cases is meant to be the slot your seal is equipped in, so right hand or left hand. The bug forces you to equip the seal in your right hand for many breath incantations to work properly while on horse, even if you can 2 hand it from left hand all the same.

Did you go to Caelid cathedral?

Just load the backup you made of your save file before you fought the boss and do it again.

Oh, wait... What am I saying? Of course you didn't make a backup of your save file, did you?

Pretty sure your stats aren’t high enough or you’re not using dragon communion. I’m built pretty tanky and an invader combod into this spell and it wrecked me

400 damage? Wow that needs nerfed, faith spells are supposed to be useless. How can I beat faith users when my halberd only hits 750 damage?

Use dragon communion seal, Dragon incantations scale primarily with Arcane if I'm not mistaken.

Just one thing to note:

Chain casting for example one Bestial Sling into this or any other chain-cast incantation seems intended and works fine. Animation transition works just fine here.

Chain casting a different incant after two Bestial Slings (or anything else) seems bugged. If you do this you'll fire off the Magma Breath with a completely wrong animation. Some spells like Dragon Claw or Black Blade bug out even worse.

I'd say it's likely the latter will get fixed eventually. The former hopefully remains untouched.

Bro the 'bugged' cast is the same speed as the nonbugged.

Can also do this with stone of gurranq and bestial sling, if you use dragon claw instead of magma breath itll use fp and do nothing though

homie I was doing invasions with this today and I was one hit twice by different weapon melee users from 1100 HP 32% damage negation. I couldn't care less if people think if me being able to do a 900 hp combo is busted.