Seems like Alluria is in control of this fight, what knowledge will Sev drop? Find out Friday! TWC image is a Bunny Girl Knight cus I really ran out of ideas.
A cantrip is a small, quick spell casting, generally of low power. They get used in combat because they are quick to cast. You really don't want to be doing a huge, elaborate spell casting that takes a few minutes to perform when somebody is charging you with a sword or lance.
She already did it once - way back near the beginning of the comic, when they were escaping from jail. Roasted a whole roomfull of guys, and then passed out from the effort, as I recall.
I am thinking that breathing fire in this way takes time and concentration to summon up the energy to let her breathe fire. Right now, she is getting distracted by her opponent and can't just stand still for a minute to summon up the energy she needs to roast somebody. That or she needs to have eaten the correct things and needs to belch methane or hydrogen which she ignites in her mouth. Neither is happening at this moment.
blow-jobsbrushing her teeth horrible!Or :D.