Well, something that has worked pretty well recently (finishing turn 1 with a single hullpoint thanks to some good dice) is fielding five units of skitarii vanguards with two arc rifles (rapid fire 1, S5, disruption) which means 10 disruption shots at first and 20 when you reach 12UM at less than 500 points. This could obviously also do wonders against Imperial Knights, Titans (except the Warlord) or simply against vehicle-heavy armies. Also because of exploration you'll cover the distance more quickly and Doctrinas Imperatives mean that you'll hit at 2+ up to three turns which means that you'll land between seven and eight hull points, which is enough to kill almost anything, if not in one turns two.
I don't know if there are more armies that can spam disruption weapons like this, but I don't think so and, if there are, probably it won't be as effective at such low cost.
Good luck at killing that tedious Imperial Knight in one turn!