If you want to insure the demise of a faction you need do nothing more than form alliances with them to in my experience usher in their future implosion.

Some might call this just a case of observed bias. But all I know is time and again, and I'm now making a specific point of doing this in every campaign, any time I see someone expand like mad, Knossos or Seleucid for example, I form defensive or military alliances and then sit back and watch the fun.

What was was once a crazily expansive faction threatening the world soon turns into a brain dead, starving, aimlessly steaming around fool of an ally, counted on to do nothing whatsoever but shrink what they control and attack zero identified diplomacy targets.

Some might call it a bug, I've gone glass half full, and I'm using it as a strategy.