Right before ( A panel or two) we see Form 3 Imu we see Killingham shocked and reacting to Imu:
Then we see Imu in his third form emanating a CoC burst:
The way I read this is the following, Imu's statement that he was forced to use too much of his power and reaching his limit means to me that he transformed in form 3 and attacked Loki right before Killinghams reaction. Leading to the scene we see where Loki is on his knees and severly injured.
If they were fighting vs form 3 the whole time Killingham wouldve sensed it earlier, which he didnt.