Easiest way to PL in this game is to quest one account like mad, get it to max level for an area, then use the apprentice system (like lackeying and sidekicking in City of Villains/Heroes) to boost your lowbies level and then run around and tear apart any mob you need to.

So far multi boxing seems a bit of a bust, although if you can get the hang of steering on 2 screens at once (usually a common move forward key, separate turn keys) you can start to reap the rewards of some classes very well, the necromancer is one of the best, with lots of pbae spells and pets able to be set to "assist me" you can just run onto a spawn and start dropping mobs.

Yet to try the group level zones, supposedly just more mob HPs, will have to look into this.