For gathering, it depends on the map, but like in Balefort Sewers Stronghold, I can loop through the first main room (provided no one else follows) and the aggro'd mobs will follow me back to where the rest of the party will nuke them. For maps like Hideout, there are places along the corridors where I run through the first set of mobs, then do a half circle at the 2nd set of mobs, which will "group" the first and 2nd set of mobs together, I hit Thorn Wall and usually that sticks them in place. You can also add Avian Scream right after Thorn Wall and that will help stun them as well. A good mage will follow and Lighting the "stuck" mobs, which combos Nature Strike, while I do the Shattering Scream+Blast Shot combo and all of those together usually takes care of group.

In general, the trick is to walk through the first group of mobs and get to the 2nd group of mobs before you Thorn Wall and/or Avian Scream. As well, sometimes I start with Shattering Scream to aggro the first line of mobs into following me up through the corridors and "lead" them to where I want to group them then Thorn Wall (and/or Avian Scream).

As for Avian Scream, I've only put 1 in it. The 3m range is a limiting factor (especially if you are pure dex), but for warbird, I think it helps because a lot of the times you are in the thick of it and the stun feature is a great way to get some breathing space while you set up for a Shattering Scream+Blast Shot combo.