tactic of armageddon using is simple.
Just use Slims defens ability - it protecs from ANY spell including Armageddon ;)

Provided you can drop the entire enemy army in 1 round with this, I think it's great. But I doubt you can do it :) On impossible you can't even drop 10% of the enemy army with this trick.

Black hole - Armageddon x2 may drop some of the army but it's the same for impossible.