A tip for fighting Jax I just discovered.
His Fatal Blow is -22 on block and has a decent pushback. I noticed that the pushback depends on how far you are from Jax once the hit meets your block: the closer the closer. So sometimes you can reach him with B12 starter but sometimes it whiffs.
I found out the best option after you block his Fatal Blow is DB4. If lands and you get KB from punish so don't hesitate to amplify.