My most important tactic is, right before pushing Namtar into the abyss, I shout "Beast from the Pit, time to feast on my s**t!" I'm not actually sure if it helps or not but it sure feels good.
More seriously, I almost always go for Binlor and heavy resistance stacking, just like you did. Namtar 1 and Namtar 2 both do a TON of magical damage. In your shoes, I would have considered using Packmaster for the Body Pact to stack even more resistance, but I can understand wanting to escape from Binlor before getting hit with the level-up piety drain.