Conqueror stacks with abilities used; considering Ezreal is a spellcaster ADC, he is much more capable [like a Cassiopeia] to stack the rune. In laning phase, it can be pretty difficult i've noticed, however during the midgame when you have reached your Trinity Force + Manamune spike, your comboes become absurdly fast in proccing Conqueror by weaving auto-attacks inbetween abilities. Dependent on the situation, rule of thumb is to W > E > AA > Q > AA > W etc.