The most competitive daemon lists, summoning or no, are about making deathstars from their fast attack and just railing through the opponents army. This means taking max size squads of units like seekers, screamers, flesh hounds, plague drones and giving them grimoire or psychic buffs like invisibility or endurance so that they are hard to remove because they are already hard hitting.
You'll just want multiple fast assault units to overwhelm your opponent and making them deathstars makes that even better. Taking a bunch of demonettess fits well into that list as well.
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