Quote from the game "The enemy's Dungeon Heart throbs before you. Assemble your minions nearby and administer the coup de grace"

So i do just that i send all of my minions of all kinds to it and they just do nothing i even put down the rally point flag and they do nothing.

This game is broken af.

You've got to put them really close to the Heart for them to start attacking. Have imps claim the ground near it.

A 'Rally Flag' or call to arms works in a particular way as well, first put it down near your creatures so that some gather around it, and then place it at the hero heart(or anywhere else) so that they follow the flag.

If nothing else, just possess a minion and kill the heart yourself.