Firewall as a skill gets a pretty bad rap here, mostly because people don’t understand how it works. And like everyone else, I noticed it kept failing in some situations, dealing considerably less damage than expected, but didn’t think much to go on. Now most people know that it’s affected by AoE defense ratio, that’s widely known, but the failure is for a different reason.
What I found out after testing is that firewall is countered by ice property monsters. I thought this was odd at first, but after testing on multiple maps against multiple monsters, ice property mobs definitely wreak havoc on your firewall. When they hit your firewall, no matter how many hits that tile has left, it will vanish after a single attack. You can take a look here: