At 58 do the hellfire quests that give you a select few blue item upgrades, this will kick you into gear level needed for a good chunk of Ramparts/Blood Furnace (Imo Furnace is a hell of a lot easier then Ramparts if you're 60).

Also with the reduced XP curve still in effect doing the main HP quests should get you to 60 or close to it.

After that, instances mainly, find the quests that give upgrades for whatever class set you're multi-boxing and make sure you do those.