[Brut]

Kill corrupts to get faction ups with non-corrupts, and visa versa.

There's also the possibility of getting both factions up, but mostly this can be tricky due to juggling with the problem of one always getting pissed at you. Some quests get one of em up without ruining the other, ie. by doing the gnoll/orc/goblin scalp quests in highpass. And there's a quest in East (or North? near the bridge) Karanas wherein you give a guy on corruptguard faction some gold for a faction up + blackburrow stout; gives faction downs with non-corrupts, but you can always fix that by initiating a gnoll genocide.

That Dagarok or Ragnarok or w/e guy roaming near the monk guild/PoD is lvl50, he's the biggest troublemaker for those kos to corrupts.