Block their block strings. Block low and wait for overheads.
If they seem to get predictable with their blockstring and you can instant block well, try for counters after IBing. It will set your opponent in the corner if you do 2D, J.D, or 5D. If you get them with 6D, just combo.
Countering is risky sometimes, wait it out and find times to either jump out, or punish them with a combo that can get you out. Jabs might help?
But if you want to play sht brainless and troll the game like I do, just press D repeaditly until you die.
Defense is the most important thing for Hakumen (and the entire cast in general), but it's arguably much more important on haku as IBing helps builds his meter that he's so dependent as well as avoiding damage (duh). Practice instant guarding and you can instant block -> counter if people go nuts on their offense without thinking. Since counters are always a risk, you can just IB near the end of their gatling where stuff is easier to punish and go for a jab and start a combo from there.