Repentia are indeed scary as.
They will all die, but Rage and a hard-to-attack priest (since the squad leader can tank a challenge) means 4 rerollable chainfist attacks each on the charge, hitting harder than terminators!
Battle conclaves haven't got much worse. Yes, all right, rhino, but you can always get out behind the rhino and drive the tank out of the way. The one thing I would advise is, mechanised or footslogging, taking a few (more) crusaders. With a 3++ and a much harder to remove feel no pain, a forward line of these boys can block a hell of a lot of heavy fire for not many points, and with the way wound allocation works, you can't assign wounds to the soft-and-squishy cultists until he's cut down every last one of the crusaders. Plus, give them a power axe and with Jacobus' bonus attack they actually become quite tasty in a fight. Equally, be sure to mix and match swords, axes and mauls in the cultists in case of terminators, ork mobz or monster units. Given that jacobus makes the unit a nasty shock charge unit - delivering an unholy number of stupidly high initiative, stupidly accurate attacks on the charge, power lances would also seem a nice investment. Wych impalers would probably work well, and it's not like many DE players use them, so they should be eminantly blag-able bitz.
Last comment - especially if sinking a lot of points into the conclave - take a priest with an eviscerator and attach him. This serves two functions; it gives you a character who can accept challenges on Jacobus' behalf if necessary (since you have no squad leader), and it allows you to put a chainfist in the unit - giving it some protection from getting stomped on by invulnerable walkers (remember you lack krak grenades)