Aquaman is nearly impossible for Batgirl.
-Ground Trident = anti-air against her jumps, beats out Batarangs on startup, can combo from a range with MB and do 20+ damage easy.
-Very hard to punish from his trident range; most of his attacks keep him safe on block
-Low Trident scoop has sick range. Can come immediately after overhead, making blocking difficult.
-In a corner, Aquaman's trident is a virtual damage wall. If you're not blocking, you will get hit and land into a combo
Batgirl has almost no punishing options. Arthur's range keeps him from being hit by DF2 on wakeup and his overhead/scoop beat the cartwheel easy. Batarangs cannot trade with ground trident, and Batgirl's normals are far too short ranged; B1 and B2 (low and overhead) cannot cover enough ground, making her completely susceptible.
7-3 sounds more like 8-2 from the Aquaman I fought (TheMaktastik 360). Jumping in gets you hit by the overhead or the uppercut, and dashing in gets you hit by the ground trident, which you can't punish. I'm 100% sure Aquaman hits the top tier, perhaps directly under Scorpion, and Batgirl will be his easiest matchup.
The one thing I've learned is, when he uses his Trait, the vortex doesn't work right. It's possible to use this to your advantage and change the combo altogether, jumping in with a 2 followed by a B1 or B2 instead of 111.
Edit: I've made a discovery. Aquaman does have a weakness; Flying Bat (DF2). On wakeup, Trident Rush makes him susceptible. You can fly right over him and deliver a combo. If he tries Trident Scoop and you're in his face, you'll go right over and he'll miss entirely, and you can just grab him before he tries something, assuming your combo can't come out quickly enough.