I actually think this one would be fine. The opposing force would just have to spread out their tanks until Megumin uses her explosion, and which point she'll be useless for the rest of the match.
Commander Erika, the St. Gloriana tank corps.I'll show you how my explosion magic will tear them to pieces☆No,
I'm sure you'd use your so-called explosion magic that you're so proud of and earn a disqualification from the judges, so you'll be an alternate.