The Imperium is weak in many areas: if I were atttempting to bring it down, I would adopt an approach of combining military "scorched earth" assaults on key agri worlds, perhaps with bioweapons, while increasing cult activity on adjacent hive and forge worlds. As the populance starts to starve, civil uprisings become all the more intense, leading to civil war, secession and chaos.

The Alpha Legion would be key to this kind of tactic, given their guerilla warfare specialty. A squad of Alpha Legionnaires in each Sector overseeing and coordinating a network of cults and agents would be more than sufficient to cause untold damage. 

Aggressive campaigns like this, properly timed to coincide with each other in dozens of adjacent sectors, would place untold stress upon the Imperium's military and economic might.

With regard to Abaddon's overall plan, perhaps it is economic in nature: he wants the Imperium to collapse in the same way the Soviet Union did, by  bankrupting it. Although traditional economics probably break down on a Galactic scale, the common people still need to be fed, or the rulers will be in trouble. The Imperium, like the Soviet Union in 1990, appears strong, but has deep cracks in its structure. Massive, constant, external pressure, both military and economic, could suddenly lead to so many individual uprisings that  the Imperium would effectively cease to function. The wider Imperium would become Balkanised, Cadia cut off without the infrastructure to support it or send reinforcements, easy prey to true military intervention on a scale only the Chaos Legions can manage. Individual Sector Lords would start to look to the defence of their own realms and refuse to send miitary aid beyond their borders. The Imperium would shatter into thousands of petty empires, chaos would reign....