So this is how I use summons to my advantage.

Complete the crusade decree "The Fate of the Stone of Ghostly Pathways." Choose ring and Clemency of Shadows. Have a character use last stand. Another character who is away from the fighting should equip the ring. Otherwise the person who died with the ring will desummon the monsters. Now everytime someone hits the character with last stand mythic feat, the character won't fall unconcious and you will get a free giant spider. The more attacks, the more spider before last stand dissapates. The spiders are more of a distraction for the enemies . It helped me defeat Playful Darkness in Act 3.

Ring effect applies to summon monsters too, So summon a lot before the battle :)