The very best way to level this that i've noticed is to swarm kill the lower strength mobs in the Xorbb group task "The Battle for Rivervale". Used 2 rogue mercs and and healer merc on burn, Warrior (Main) and a cleric and sk box for support. At lvl 18 I got 50% in 30 min. Key is to kill as fast as u can going north of the wall from the NPC's. Kill as many as u can before the NPC's start killing them. If the repops (which are much stronger) start spawning on u, just drop task and reget, but you should be able to swarm kill for 30 min easy w/o much issue. If u bring live players and not mercs i'm sure u'd out kill the npcs.
Also swarm killed at the top of MG instanced, 5% per clear, very little effort and good plat for ur time. But if ur looking to max helm in a day or 2, Xorbb swarm is the best.