Killing dragon gives you 5 stacks but assisting in killing it only gives you one stack. Not sure if this is the same for baron or not. Pretty sure assisting in the dragon secure is supposed to give you 5 even if you don't last hit it, or at least thats what the patch notes said. If I'm wrong on that then "My B".
Actually, I had a case where immediately after buying devourer I solo'd a dragon, rift scuttler, crimson raptor, enemy jungler, red brambleback, and ancient krug... which should have given me 5(dragon) + 2(scuttler) + 4x1 (for each of the others), but I only had 6 stacks, which makes me wonder if stacking works right the first time around, at least on 'epic' monsters like dragon...
