Are the rivals attacking them? Do the PCs have to defend their haven? (sorry, wasn't clear from the question)
What is the objective of the badguys? Just to drive them off, intimidate them, or to actually wipe them out?
I'd say you need to emphasise to the players exactly how screwed they are. Rules as written, in a straight fight, they have absolutely zero chance of beating 5 to 1 odds.
However, assuming they have time to prep, and use it, and are clever, you could give the opportunity to stack the deck in their favour by cleverly preparing traps, and fortifying their haven, and using the results of their prep skill tests to thin the enemy number come fight time.
Also, if you don't want it to result in a TPK, make sure they've got the chance to run away, either before or during the fight.
In this scenario the pc's have decided to strike first as the blackhearts have been searching for their haven and want to fight the blackhearts on their own terms.
Objective of the blackhearts is to take from anyone they come across so its super unlikely they would let it end peacefully.
Escape was discussed but there are two swarm size 6 walker hordes very close to their haven. (They have had some bad lucknon random generation rolls. ) They are worried they will die fighting a large swarm and want to go out fighting the blackhearts as they have a better chance. Then they will deal with the giant hordes.)
Ah, so if they're attacking the Blackhearts, then I would assume your players aren't gonna be dumb enough to just do a full frontal assault.
So I guess you just let them scout out and recon their base, and come up with a plan to Seal Team 6 them, picking off pockets of enemies, ambushing patrols out in the wilderness, etc...
And if they don't, and they screw it up? Well, more fool them, lol.