Going under them just means you could have done an extra Auto AA or two when you didn't. Melee will hit at any height, but the vast majority (actually all, I think) of Dainn skills have a target height range.
The part of autoing in a wall combo is that you actually have to auto slowly (but not TOO slowly) and in a back and forth pattern (ie 776 116 776) to achieve the maximum number of hits. You don't want to get too close to the enemy, as nothing will hit then. When you auto slowly, you move back a couple of pixels before hitting them again.
Fire Bomb can be Auto AA'd twice times (or HBD AA'd) before Dash ASing. Personally, I use Pillar of Abyss after Fire Bomb because it also has a short (10 second) cooldown and it re-establishes some height on the target.