Seems to me that one would be able to kill Bellach right after finishing Quest 1B.
After you do so, that quest becomes inactive and the new active quest is Quest 2A which reads that Bellach makes an immediate attack; though at no point on Quest 2A does it say that Bellach is immune to player cards, so a combination of Spear of the Citadel, Gondorian Spearman and/or Swift Strike (maybe others) should be able to bring him down at this point, seeing as neither Quest 1B nor 2B (which contain the text making Bellach immune) are active at the moment he is making his attack.
Would this not be valid?
Apologies if this has been discussed earlier.