Whirlwind on the Soul Seeker is exactly what I use to get the keys for Travesty, especially the red key. It pretty much wipes out everything. You just have to watch out when you start to run low on enemies to hit, and switch over to a 1-1 combat so you can leech enough. Targets tend to wander off, and you don't want to roam around too much or you'll just respawn a bunch more that you have to kill, so it's good to get as many life bars up as you can, then switch targets as necessary.