Hey Isaac.
Some people already mentioned that some D2s work, as well as dashing under him and attacking the other way, which is true.
Another strategy could be MBF3ing his J3 on reaction. Green Arrow's seems pretty slow but it could probably work if its preemptive. Other characters with better F3s such as Green Lantern, Aquaman, Deathstroke, Killer Frost, etc. have more leeway in doing this.
Another strategy could be dashing back or maybe even walking back on his J3 coming down and then attacking him from there. It may or may not punish him on a tripguard. I'm not sure how good Green Arrow is at taking advantage of this.
Try going to training mode and record Deathstroke doing J3 over and over. Make him also hold down so when he comes to the ground, he blocks as soon as he can. From there, see what you can come up with.