The problem with trying to separate the HK from Imu is that their connection to Imu is the entire point. He can communicate telepathically, use his powers remotely to great effect as we saw ruin Rocks, and teleport in if things are bad enough.
The hypothetical of beating Killingham without Omen or Imu not doing anything... that chance is gone in the story. As readers, at the very least we know that this was never on the table anyway