"Ha-ha! Thanks for the lift, Ember, I should be fine from here!" THOOMP! "What in the-augh! Ack, grrngh, hack, w-whachough-sniffle Is… was that frickin'gag-cough frickin' DRAGONSNEEZE!? What the" THOOMP! "Augh!" "Yeah, who's the top dragon now, huh? You're not so high-and-mighty when I've got a couple'a toys my friends cooked up, huh?" "Yousneeze-cough-hack ya little twerp! I'm gonna-" THOOMP! "Ack, my eyes!"
What's the best answer for how to deal with an annoying dragon now you've got something that renders you immune to dragonsneeze? Weaponizing the plant, naturally! Stick it in a few 40mm-sized balls with some self-contained propellant charges, get an airlift from a friend, and start getting to work giving Garble a severe allergic reaction.
Don't worry, Spike's got diplomatic immunity, not like they're gonna start a war over a localized use of a biological agent. That and Garble was asking for it.