I've did it before, so I'll do it again. This time it'll be a group of death cultists and their leaders. Hopefully someone will find of use and even if they don't, I'll probably use it somewhere.
1 Acolyte (Bless, Cure Wound, Inflict Wound) *
* Can be increased to 2 if greater challenge is needed.
** Find stats for both here.
The cultists will swarm hostile targets and attack until they get other orders. The leaders will try to keep their cultists alive but also keep one spell slot in reserve for an inflict wound. Using the screen of their cultists it allows the acolyte leaders to come within range. If the fight is clearly lost, the acolytes will try to escape while leaving their cultists to hold up the enemy. They could also try to gather reinforcements and come back if such an option presents itself.
The size of the cult should be planned ahead of time, I think, as well as the type of cult. Doing so allows for possible reinforcements to be planned as well as story points. The loss of part of a larger cult could result in hunting parties to find out what happened, who did it and to avenge the loss. It also helps determine the role of the cult in a campaign. A small one may play a small role in one particular part. A bigger cult could provide for resistance through the entire campaign.
One cult could have good or bad relations with another or with established and well accepted religions. This could mean that the players gain unexpected allies through their conflict with the cultists. They could also gain unexpected enemies in the same way. This could be more complicated as well. A group may be against the cultists, but also against the players because of how they handled the situations they encountered the cultists in. The loss of an acolyte could also set plans back days, weeks, or even months depending on the importance of the figure.
Things Not Going as Planned
Decide before hand if the group is so devoted to their cause that they will not retreat, or if they will retreat if their leader(s) are killed. When this occurs, they will retreat without any roll being made. Otherwise, they will continue to push forward unless an order made by their leader(s) tells them otherwise. Depending on the kind of cult, the leader may be the most important consideration. In other cases, a particular item could be fallen back to and defended instead (in such a case, the leader would be less important than the goal of the group).