Damaar was fighting both Infernus AND Bagan at the same time. in a one on one fight, I very much doubt Infernus would get the chance to get a good shot in

Plus, Damaar's completely immune to Infernus' weapons. to hurt him at all you need to take him on in melee. and Damaar in a 1 v 1 fight, in melee = death

I actually had several opportunities to smash Infernus' jaws, legs and ribs to the point his own body could kill him. I avoided it though for the sake of fun.
Don't hope on the same mercy next time. Hell, I wouldn't get my hopes up just yet. Damaar's a lot more intelligent than his appearance implies. He did kill his home planet, afterall [you'll learn more about that once Horus finds out he's here]