These creatures are hard to tame at low level because they have a swiping move that interrupts taming. You might try using Scare Beast (although you'll need to be clever how you position yourself, or the ravager will run out of range). Or you can wait until you get Frost Trap at level 20, then use a Concussive Shot to pull them slowly into the trap. You may also find it helpful to start the tame from the far side of a tree - running around obstacles will also slow them down and buy you a few seconds. There are also a few items that can help, such as Slumber Sand or Magic Dust.