Skeletons are vulnerable while emerging from the ground. There's a party in the graveyard, and the Skeletons just keep coming back.
Tank + AoE Minis can take care of the Ghouls and Skeletons that spawn. Unbound Minis like Whelp Eggs can take care of the Abomination Minis.
For every PvE mission, there is a campaign version and a Heroic version. When you first begin the game, you can only participate in campaign missions. Upon collecting 50 sigils, you will then unlock Heroic missions.
Campaign missions will grant rewards after the first successful clear with ANY family. Heroic missions must be completed with EACH of the 5 different families in order to receive the mission rewards.