A simple answer to Classes Problem/hitbox on Defensive characters

To add survivability to the tanks, how about this:

Gibraltar - make the passive weapon shield less colorful and stretch it so he is covered fully from front, yet not more visible from afar.

Caustic - add to his passive one more thing - after every shot hits him, he releases a small amout of gas (damage related), so after say 100 incoming damage he will be almost fully covered in gas, pretty much not visible in that cloud.