Considering how easily default Mechas and Cannonballs and even Side 3 can wreck Flamethrower, it's nice to read that Zard has an alternative in the form of Fireball Cannon. While it can disrupt many of Jr's moves such as his default Mechas & Cannonballs during their startup, and Side 1 & 2 in general, it won't be able to disrupt Jr's Side 3 too well, so if Jr's annoyed enough by Zard's Neutral 3, he might switch to Side 3 and/or even Up 2 in retaliation.
The 1st 3 huffs of FC are enough to KO default Mecha, but you'll need some distance (perhaps a little less than 1/2 the distance of an Omega? I'd say 1/2 the distance to be safe.) for all 3 huffs to come out, and it requires the most commitment of the 3 Neutral B moves. If the 3rd huff misses the Mecha, you'll likely be getting the full brunt of the explosive impact, and if Jr's ready for that moment, he can chase with Side B for the followup.
FC can also safely clank with tapped/uncharged Cannonballs, but it'll lose to charged ones, which will pierce through and hit you for full damage and knockback.
FC can still gimp Jr post Up 1/2 (from a further distance too!), but you'll need somewhat better timing/luck to pull it off than you would need with Flamethrower. To ensure that you're aiming FC downwards (which does reach further downwards than Neutral 1), hold down while holding B. Even if you don't gimp him with FC, at least you'll be able to snag some free damage from further away than Neutral 1 can.
I'm pleasantly surprised to be liking Fireball Cannon enough to recommend it over Flamethrower for this MU. I would've dared suggest Fire Fang, but giving up the ability to scare Jrs from recovering low might be too much to ask. Thus, I'd recommend 3311 or 3313 over 1311 or 1313. In fact, I'd personally run 3313 myself, were I to be Zard in this MU, going up against a Side B JC-ing Jr whose UAir sharking abilities are on point.